Fix missing LIBPROTOC_EXPORT.
Fix missing LIBPROTOC_EXPORT.

This makes possible to build grpc_php_plugin with protobuf in shared mode.
google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf