os: add missing copyright notice

Change-Id: I496b701e2fcc944c764002643c7b0b2ce9e08806
Reviewed-on: https://go-review.googlesource.com/18182Reviewed-by: Dave Cheney <dave@cheney.net>

src/os/os_windows_test.go

+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
 package os_test
 
 import (