internal/testprotos: add a missing Copyright header line

Change-Id: Idb92e34af7c5342e4c4478707eb99fcb824d7606
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/220350
Reviewed-by: Joe Tsai <joetsai@google.com>
diff --git a/internal/testprotos/order/order.pb.go b/internal/testprotos/order/order.pb.go
index 82da29f..07fa658 100644
--- a/internal/testprotos/order/order.pb.go
+++ b/internal/testprotos/order/order.pb.go
@@ -1,3 +1,4 @@
+// Copyright 2019 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.
 
diff --git a/internal/testprotos/order/order.proto b/internal/testprotos/order/order.proto
index 2a214dd..9333ace 100644
--- a/internal/testprotos/order/order.proto
+++ b/internal/testprotos/order/order.proto
@@ -1,3 +1,4 @@
+// Copyright 2019 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.