godoc: add missing copyright

Change-Id: Ic0256b511b4d4346702aeac18298614b1e8d5438
Reviewed-on: https://go-review.googlesource.com/9384
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/godoc/tab.go b/godoc/tab.go
index 7973b74..d314ac7 100644
--- a/godoc/tab.go
+++ b/godoc/tab.go
@@ -1,3 +1,7 @@
+// Copyright 2013 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.
+
 // TODO(bradfitz,adg): move to util
 
 package godoc