Add an objc_class_prefix for the proto2 syntax file used in conforman…

…ce tests.

Not all the types have "Proto2" on the end, so for uniformity put an
objc_class_prefix like was done for the proto3 syntax file.

google/protobuf/test_messages_proto2.proto

@@ -40,6 +40,7 @@ syntax = "proto2";
 package protobuf_test_messages.proto2;
 
 option java_package = "com.google.protobuf_test_messages.proto2";
+option objc_class_prefix = "Proto2";
 
 // This is the default, but we specify it here explicitly.
 option optimize_for = SPEED;