Use 'ignore-' instead of 'only-'

Apparently 'ignore-' doesn't work with compiletest_rs

Author: Aaron1011

tests/compile-fail/getrandom.rs

@@ -1,4 +1,5 @@
-// only-linux
+// ignore-macos
+// ignore-windows
 
 #![feature(rustc_private)]
 extern crate libc;