fix libc feature gate

Author: RalfJung

tests/run-pass/foreign-fn-linkname.rs

@@ -10,7 +10,7 @@
 
 //ignore-windows: Uses POSIX APIs
 
-#![feature(libc)]
+#![feature(rustc_private)]
 #![allow(unused_extern_crates)] // rustc bug https://github.com/rust-lang/rust/issues/56098
 
 extern crate libc;

tests/run-pass/regions-mock-trans.rs

@@ -10,7 +10,7 @@
 
 //ignore-windows: Uses POSIX APIs
 
-#![feature(libc)]
+#![feature(rustc_private)]
 
 #![allow(dead_code)]
 

tests/run-pass/thread-local.rs

@@ -1,6 +1,6 @@
 //ignore-windows: Uses POSIX APIs
 
-#![feature(libc)]
+#![feature(rustc_private)]
 extern crate libc;
 
 use std::mem;