fix: bring back feature(rustc_private)

error: cannot satisfy dependencies so `panic_unwind` only shows up once
  |
  = help: having upstream crates all available in one format will likely make this go away
  = help: `feature(rustc_private)` is needed to link to the compiler's `rustc_driver` library

rap/src/bin/cargo-rap/main.rs

@@ -2,6 +2,7 @@
     This is a cargo program to start RAP.
     The file references the cargo file for Miri: https://github.com/rust-lang/miri/blob/master/cargo-miri/src/main.rs
 */
+#![feature(rustc_private)]
 
 #[macro_use]
 extern crate rap;