wgengine: fix windows build for winipcfg-go

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/rtnlmon/mon.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 !windows
+
 // Package rtnlmon watches for "interesting" changes to the network
 // stack and fires a callback.
 package rtnlmon