only build benches/kernel_swap on Linux

benches/kernel_swap.rs

@@ -1,6 +1,7 @@
 // This file is part of libfringe, a low-level green threading library.
 // Copyright (c) 2015, edef <edef@edef.eu>
 // See the LICENSE file included in this distribution.
+#![cfg(target_os = "linux")]
 #![feature(asm, test)]
 extern crate test;
 use test::Bencher;