syscall: make getrlimit, setrlimit tests work on unix variants

R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/53690043

src/pkg/syscall/rlimit_unix_test.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin dragonfly freebsd linux netbsd openbsd
+
 package syscall_test
 
 import (