add syntax statement since protoc warns to declare explicitly (#410)

managed-ledger/src/main/proto/MLDataFormats.proto

@@ -13,6 +13,8 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+syntax = "proto2";
+
 option java_package = "org.apache.bookkeeper.mledger.proto";
 option optimize_for = SPEED;
 

pulsar-common/src/main/proto/PulsarApi.proto

@@ -13,6 +13,8 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+syntax = "proto2";
+
 package pulsar.proto;
 option java_package = "com.yahoo.pulsar.common.api.proto";
 option optimize_for = LITE_RUNTIME;