Bug 1411450 - Add the protobuf version to LayerScopePacket.proto. r=mattwoodrow

This is essentially a no-op but it does silence the following warning
whenever the *.pb.* files are re-generated:

[libprotobuf WARNING google/protobuf/compiler/parser.cc:546] No syntax specified for the proto file: LayerScopePacket.proto. Please use 'syntax = "proto2";' or 'syntax = "proto3";' to specify a syntax version. (Defaulted to proto2 syntax.)

MozReview-Commit-ID: 6L1TXHLcjsj

--HG--
extra : rebase_source : c88211414242c921eeefec27476c4a52e6c0974a

gfx/layers/protobuf/LayerScopePacket.proto

@@ -1,5 +1,7 @@
 /* vim:set ts=2 sw=2 sts=2 et: */
 
+syntax = "proto2";
+
 option optimize_for = LITE_RUNTIME;
 
 package mozilla.layers.layerscope;