Enable the -Wundef warning for clang

webrtc/test/rtp_file_writer_unittest.cc

 /*
  *  Copyright (c) 2014 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 #include <string.h>
 
 #include <memory>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/test/rtp_file_reader.h"
 #include "webrtc/test/rtp_file_writer.h"
 #include "webrtc/test/testsupport/fileutils.h"
 
 namespace webrtc {
 
 class RtpFileWriterTest : public ::testing::Test {
  public:
   void Init(const std::string& filename) {
     filename_ = test::OutputPath() + filename;
@@ skipping 41 matching lines @@
 };
 
 TEST_F(RtpFileWriterTest, WriteToRtpDump) {
   Init("test_rtp_file_writer.rtp");
   WriteRtpPackets(10);
   CloseOutputFile();
   VerifyFileContents(10);
 }
 
 }  // namespace webrtc