Appease the "missing copyright header" check.

Signed-off-by: Avery Pennarun <[email protected]>

control/controlclient/filter.go

@@ -1,3 +1,7 @@
+// Copyright (c) 2020 Tailscale Inc & 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 controlclient
 
 import (