gnupg: use --with-readline configure flag

as readline is keg-only, gnupg cannot find the appropriate libraries
when building. this results in a linker error when building on arm64
macos. providing the correct path to readline allows this formula to
build without issue.

Formula/g/gnupg.rb

@@ -54,6 +54,7 @@ def install
                              "--enable-all-tests",
                              "--sysconfdir=#{etc}",
                              "--with-pinentry-pgm=#{Formula["pinentry"].opt_bin}/pinentry",
+                             "--with-readline=#{Formula["readline"].opt_prefix}",
                              *std_configure_args
       system "make"
       system "make", "check"