Fix warnings during tests

The deny(warnings) attribute is now enabled for tests so we need to weed out
these warnings as well.

src/libstd/sys/unix/stack_overflow.rs

@@ -7,6 +7,7 @@
 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
+
 #![cfg_attr(test, allow(dead_code))]
 
 use libc;