GN: Fix iOS error in audio_device and rtc_base

webrtc/modules/audio_device/BUILD.gn

 # 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.
 
 import("../../build/webrtc.gni")
 
@@ skipping 32 matching lines @@
   if (is_mac) {
     include_dirs += [ "mac" ]
   }
   if (is_win) {
     include_dirs += [ "win" ]
   }
   if (is_android) {
     include_dirs += [ "android" ]
   }
   defines = []
+  cflags = []
   if (rtc_include_internal_audio_device) {
     defines += [ "WEBRTC_INCLUDE_INTERNAL_AUDIO_DEVICE" ]
     sources += [
       "android/audio_device_template.h",
       "android/audio_manager.cc",
       "android/audio_manager.h",
       "android/audio_record_jni.cc",
       "android/audio_record_jni.h",
       "android/audio_track_jni.cc",
       "android/audio_track_jni.h",
@@ skipping 97 matching lines @@
   }
 
   deps = [
     "../..:webrtc_common",
     "../../base:rtc_base_approved",
     "../../common_audio",
     "../../system_wrappers",
     "../utility",
   ]
 }