Ignore other platforms instead of using only-linux

Author: Aaron1011

tests/run-pass/linux-getrandom.rs

@@ -1,5 +1,7 @@
-// only-linux: Uses Linux-only APIs
-
+// Unfortunately, compiletest_rs does not support 'only-linux',
+// so we need to ignore Windows and OS X instead
+// ignore-macos: Uses Linux-only APIs
+// ignore-windows: Uses Linux-only APIs
 #![feature(rustc_private)]
 extern crate libc;