update_engine: Defines proto2 as the syntax of the protobuf

Adds 'syntax = "proto2"; to update_metadata.proto to shut up the
protobuf compiler about the default version of the protobuf.

BUG=none
TEST=cros_workon_make --board=amd64-generic --test update_engine

Change-Id: If1689825f1296163ae81c59581fa00dad435fdce
Reviewed-on: https://chromium-review.googlesource.com/602692
Commit-Ready: Amin Hassani <[email protected]>
Tested-by: Amin Hassani <[email protected]>
Reviewed-by: Ben Chan <[email protected]>
diff --git a/update_metadata.proto b/update_metadata.proto
index 454c736..f86cd8d 100644
--- a/update_metadata.proto
+++ b/update_metadata.proto
@@ -81,6 +81,8 @@
 // The operations allowed in the payload (supported by the client) depend on the
 // major and minor version. See InstallOperation.Type bellow for details.
 
+syntax = "proto2";
+
 package chromeos_update_engine;
 option optimize_for = LITE_RUNTIME;