Add an objc_class_prefix for the proto2 syntax file used in conformance 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.

conformance/conformance_objc.m

diff --git a/conformance/conformance_objc.m b/conformance/conformance_objc.m
index ab751c8..12f06c9 100644
--- a/conformance/conformance_objc.m
+++ b/conformance/conformance_objc.m
@@ -77,7 +77,7 @@
       if ([request.messageType isEqual:@"protobuf_test_messages.proto3.TestAllTypesProto3"]) {
         msgClass = [Proto3TestAllTypesProto3 class];
       } else if ([request.messageType isEqual:@"protobuf_test_messages.proto2.TestAllTypesProto2"]) {
-        msgClass = [TestAllTypesProto2 class];
+        msgClass = [Proto2TestAllTypesProto2 class];
       } else {
         response.runtimeError =
             [NSString stringWithFormat:@"Protobuf request had an unknown message_type: %@",