Restore empty starboard_poem.h

Evidently this file name is hard-coded into the binary tools/protoc_mac
added by Change Ib2045ae4.

Issue: 206653074
Issue: 207292694

third_party/protobuf/src/google/protobuf/stubs/starboard_poem.h

@@ -0,0 +1,13 @@
+// Copyright 2019 The Cobalt Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.