Enable the -Wundef warning for clang

webrtc/modules/rtp_rtcp/source/ulpfec_header_reader_writer_unittest.cc

 /*
  *  Copyright (c) 2016 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 <utility>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/base/basictypes.h"
 #include "webrtc/base/checks.h"
 #include "webrtc/base/random.h"
 #include "webrtc/base/scoped_ref_ptr.h"
 #include "webrtc/modules/rtp_rtcp/source/byte_io.h"
 #include "webrtc/modules/rtp_rtcp/source/forward_error_correction.h"
 #include "webrtc/modules/rtp_rtcp/source/forward_error_correction_internal.h"
 #include "webrtc/modules/rtp_rtcp/source/ulpfec_header_reader_writer.h"
 
 namespace webrtc {
@@ skipping 208 matching lines @@
   auto packet_mask = GeneratePacketMask(packet_mask_size, 0xabcd);
 
   auto written_packet = WriteHeader(packet_mask.get(), packet_mask_size);
   auto read_packet = ReadHeader(*written_packet);
 
   VerifyHeaders(kUlpfecHeaderSizeLBitSet, packet_mask.get(), packet_mask_size,
                 *written_packet, *read_packet);
 }
 
 }  // namespace webrtc