Add missing golden test file.

src/Makefile.am

@@ -530,6 +530,7 @@ EXTRA_DIST =                                                   \
   google/protobuf/io/gzip_stream.h                             \
   google/protobuf/io/gzip_stream_unittest.sh                   \
   google/protobuf/testdata/golden_message                      \
+  google/protobuf/testdata/golden_message_maps                 \
   google/protobuf/testdata/golden_message_oneof_implemented    \
   google/protobuf/testdata/golden_message_proto3               \
   google/protobuf/testdata/golden_packed_fields_message        \