GN: Change rtc_source_set targets --> rtc_static_library

webrtc/modules/audio_mixer/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")
 
 config("audio_conference_mixer_config") {
   include_dirs = [ "../../modules/include" ]
 }
 
-rtc_source_set("audio_mixer") {
+rtc_static_library("audio_mixer") {
   sources = [
     "audio_frame_manipulator.cc",
     "audio_frame_manipulator.h",
     "audio_mixer.h",
     "audio_mixer_defines.h",
     "audio_mixer_impl.cc",
     "audio_mixer_impl.h",
   ]
 
   public = [
     "audio_mixer_defines.h",
     "audio_mixer.h",
   ]
 
   public_configs = [ ":audio_conference_mixer_config" ]
 
   deps = [
     "../..:webrtc_common",
     "../../base:rtc_base_approved",
     "../../modules/audio_processing",
     "../../modules/utility",
     "../../system_wrappers",
     "../../voice_engine:level_indicator",
   ]
 }