chore: add license to BUILD file

Author: vikerman

third_party/github.com/bazelbuild/bazel/src/main/java/com/google/devtools/build/lib/buildeventstream/proto/BUILD

@@ -1,3 +1,7 @@
+# Copyright Google Inc. All Rights Reserved.
+#
+# Use of this source code is governed by an MIT-style license that can be
+# found in the LICENSE file at https://angular.io/license
 package(default_visibility = ["//:__subpackages__"])
 
 licenses(["notice"])  # Apache 2.0