Whitelist unwind import in std

This is a part of libbacktrace linkage and thus the compiler cannot detect if it's used or not.

Author: ishitatsuyuki

src/libstd/lib.rs

@@ -283,7 +283,7 @@
 #![feature(on_unimplemented)]
 #![feature(oom)]
 #![feature(optin_builtin_traits)]
-#![cfg_attr(any(unix, target_os = "redox"), feature(panic_unwind))]
+#![feature(panic_unwind)]
 #![feature(peek)]
 #![feature(placement_in_syntax)]
 #![feature(placement_new_protocol)]
@@ -358,7 +358,7 @@ extern crate std_unicode;
 extern crate libc;
 
 // We always need an unwinder currently for backtraces
-#[cfg(any(unix, target_os = "redox"))]
+#[allow(unused_extern_crates)]
 extern crate unwind;
 
 // compiler-rt intrinsics