go/internal/gccgoimporter: add missing copyright notice

Change-Id: I20f7aba8a76cfe8549771ef96244e8641a4157e6
Reviewed-on: https://go-review.googlesource.com/c/142891
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/go/internal/gccgoimporter/backdoor.go b/go/internal/gccgoimporter/backdoor.go
index 4feb0dc..533fc03 100644
--- a/go/internal/gccgoimporter/backdoor.go
+++ b/go/internal/gccgoimporter/backdoor.go
@@ -1,3 +1,7 @@
+// Copyright 2018 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 gccgoimporter
 
 // This file opens a back door to the parser for golang.org/x/tools/go/gccgoexportdata.