Fix modules_unittests on iOS.

webrtc/modules/BUILD.gn

Index: webrtc/modules/BUILD.gn
diff --git a/webrtc/modules/BUILD.gn b/webrtc/modules/BUILD.gn
index 89959340d9d4ba3c16c31b222643aebeb3c66d2d..c7db7b7dd9d834837f6c8563eac0dd606db4d8dd 100644
--- a/webrtc/modules/BUILD.gn
+++ b/webrtc/modules/BUILD.gn
@@ -667,11 +667,14 @@ if (rtc_include_tests) {
       ]
 
       sources += [
-        "audio_device/ios/audio_device_unittest_ios.cc",
         "audio_device/ios/objc/RTCAudioSessionTest.mm",
         "video_coding/codecs/h264/h264_video_toolbox_nalu_unittest.cc",
       ]
 
+      if (target_cpu != "x64") {
+        sources += [ "audio_device/ios/audio_device_unittest_ios.cc" ]
+      }
+
       ldflags = [ "-ObjC" ]
     }
   }