windows: make all files in the package require windows

env_unset.go didn't have a the windows build tag (or filename), so it
was compiling for all operating systems and as a result had weird docs
at https://godoc.org/golang.org/x/sys/windows showing only Unsetenv.

Updates golang/gddo#188

Change-Id: I58f2c27c3d267660c97fce5a1b28da24988c62c8
Reviewed-on: https://go-review.googlesource.com/21460
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

windows/env_unset.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build windows
 // +build go1.4
 
 package windows