Enable the -Wundef warning for clang

webrtc/test/rtp_file_reader_unittest.cc

 /*
  *  Copyright (c) 2013 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 <map>
 #include <memory>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/modules/rtp_rtcp/source/rtp_utility.h"
 #include "webrtc/test/rtp_file_reader.h"
 #include "webrtc/test/testsupport/fileutils.h"
 
 namespace webrtc {
 
 class TestRtpFileReader : public ::testing::Test {
  public:
   void Init(const std::string& filename, bool headers_only_file) {
     std::string filepath =
@@ skipping 92 matching lines @@
 
 TEST_F(TestPcapFileReader, TestThreeSsrc) {
   Init("ssrcs-3");
   PacketsPerSsrc pps = CountRtpPacketsPerSsrc();
   EXPECT_EQ(3UL, pps.size());
   EXPECT_EQ(162, pps[0x938c5eaa]);
   EXPECT_EQ(113, pps[0x59fe6ef0]);
   EXPECT_EQ(61, pps[0xed2bd2ac]);
 }
 }  // namespace webrtc