Fix build for iOS

[maun]

Tested using cargo-mobile2.

Works with a simple rect:

empty().style(|s| s.size_pct(70.0, 30.0).background(Color::RED))

but crashes when using text due to a panic in cosmic-text.

window creation done
window event Focused(true)
thread '<unnamed>' panicked at /Users/manuel/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cosmic-text-0.12.1/src/shape.rs:251:33:
no default font found
stack backtrace:
   0:        0x107505b24 - std::backtrace_rs::backtrace::libunwind::trace::h2504f6a7a9083436
                               at /rustc/90b35a6239c3d8bdabc530a6a0816f7ff89a0aaf/library/std/src/../../backtrace/src/backtrace/libunwind.rs:116:5
   1:        0x107505b24 - std::backtrace_rs::backtrace::trace_unsynchronized::h6d30c5a7e5c0e95b
                               at /rustc/90b35a6239c3d8bdabc530a6a0816f7ff89a0aaf/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:        0x107505b24 - std::sys::backtrace::_print_fmt::h4d241177db3ad3db
                               at /rustc/90b35a6239c3d8bdabc530a6a0816f7ff89a0aaf/library/std/src/sys/backtrace.rs:66:9
   3:        0x107505b24 - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::h7a0e784acb8aeff7
                               at /rustc/90b35a6239c3d8bdabc530a6a0816f7ff89a0aaf/library/std/src/sys/backtrace.rs:39:26
   4:        0x107520730 - core::fmt::rt::Argument::fmt::hdcf4364f643ba5f0
                               at /rustc/90b35a6239c3d8bdabc530a6a0816f7ff89a0aaf/library/core/src/fmt/rt.rs:177:76
   5:        0x107520730 - core::fmt::write::hc7ae0c33f591ced7
                               at /rustc/90b35a6239c3d8bdabc530a6a0816f7ff89a0aaf/library/core/src/fmt/mod.rs:1186:21
   6:        0x107503224 - std::io::Write::write_fmt::h73a91b3d4edc8b40
                               at /rustc/90b35a6239c3d8bdabc530a6a0816f7ff89a0aaf/library/std/src/io/mod.rs:1839:15
   7:        0x1075059d8 - std::sys::backtrace::BacktraceLock::print::ha96d44dc3d9057c0
                               at /rustc/90b35a6239c3d8bdabc530a6a0816f7ff89a0aaf/library/std/src/sys/backtrace.rs:42:9
   8:        0x1075068fc - std::panicking::default_hook::{{closure}}::h6410df3ad7c0548b
                               at /rustc/90b35a6239c3d8bdabc530a6a0816f7ff89a0aaf/library/std/src/panicking.rs:268:22
   9:        0x107506744 - std::panicking::default_hook::h04f2d093d7cf1671
                               at /rustc/90b35a6239c3d8bdabc530a6a0816f7ff89a0aaf/library/std/src/panicking.rs:295:9
  10:        0x1075070b4 - std::panicking::rust_panic_with_hook::h5564521c80b92207
                               at /rustc/90b35a6239c3d8bdabc530a6a0816f7ff89a0aaf/library/std/src/panicking.rs:801:13
  11:        0x107506d64 - std::panicking::begin_panic_handler::{{closure}}::h24bfefb8a0ebe306
                               at /rustc/90b35a6239c3d8bdabc530a6a0816f7ff89a0aaf/library/std/src/panicking.rs:674:13
  12:        0x107505fe8 - std::sys::backtrace::__rust_end_short_backtrace::hc1a47cedc59d63b1
                               at /rustc/90b35a6239c3d8bdabc530a6a0816f7ff89a0aaf/library/std/src/sys/backtrace.rs:170:18
  13:        0x107506a1c - rust_begin_unwind
                               at /rustc/90b35a6239c3d8bdabc530a6a0816f7ff89a0aaf/library/std/src/panicking.rs:665:5
  14:        0x10755e730 - core::panicking::panic_fmt::hcf86678192bed007
                               at /rustc/90b35a6239c3d8bdabc530a6a0816f7ff89a0aaf/library/core/src/panicking.rs:74:14
  15:        0x10755e710 - core::panicking::panic_display::hab223dafa50efa5e
                               at /rustc/90b35a6239c3d8bdabc530a6a0816f7ff89a0aaf/library/core/src/panicking.rs:264:5
  16:        0x10755e710 - core::option::expect_failed::hc0aae6e2775950e6
                               at /rustc/90b35a6239c3d8bdabc530a6a0816f7ff89a0aaf/library/core/src/option.rs:2021:5
  17:        0x10706e080 - core::option::Option<T>::expect::h61e77952fb293d2e
                               at /Users/manuel/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/src/rust/library/core/src/option.rs:933:21
  18:        0x107099d94 - cosmic_text::shape::shape_run::hbb59a34151ce1658
                               at /Users/manuel/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cosmic-text-0.12.1/src/shape.rs:251:16
  19:        0x10709ac50 - cosmic_text::shape::shape_run_cached::h062accfdf377c51c
                               at /Users/manuel/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cosmic-text-0.12.1/src/shape.rs:405:5
  20:        0x107098c58 - cosmic_text::shape::Shaping::run::h5d3b31157047d2d9