Fix build: replace `proto2::` namespace with `tensorflow::protobuf::` for portability (NFC)

PiperOrigin-RevId: 394265326
Change-Id: I95316bcfa661112f5ca14981a85a7d647a593e51
diff --git a/tensorflow/core/ir/importexport/tests/roundtrip/roundtrip.cc b/tensorflow/core/ir/importexport/tests/roundtrip/roundtrip.cc
index 93b090a..630de47 100644
--- a/tensorflow/core/ir/importexport/tests/roundtrip/roundtrip.cc
+++ b/tensorflow/core/ir/importexport/tests/roundtrip/roundtrip.cc
@@ -169,8 +169,8 @@
   }
   NormalizeTensorData(new_graph);
   NormalizeTensorData(original_graph);
-  if (!proto2::util::MessageDifferencer::Equivalent(original_graph,
-                                                    new_graph)) {
+  if (!tensorflow::protobuf::util::MessageDifferencer::Equivalent(
+          original_graph, new_graph)) {
     LOG(ERROR) << "GraphDef didn't Roundtrip:";
     llvm::errs()
         << "\n=========\n\n"