Silence warnings about unstable feature core

Author: mvdnes

src/lib.rs

@@ -21,7 +21,7 @@
 //! the system libc library.
 
 #![no_std]
-#![allow(unstable)]
+#![feature(core)]
 
 // This library defines the builtin functions, so it would be a shame for
 // LLVM to optimize these function calls to themselves!