Added new mixer folder to CPPLINT_DIRS. Fixed one small issue.

NOTRY=True

Review-Url: https://codereview.webrtc.org/2228503003
Cr-Commit-Position: refs/heads/master@{#13677}

PRESUBMIT.py

@@ -20,6 +20,7 @@ CPPLINT_DIRS = [
   'webrtc/call',
   'webrtc/common_video',
   'webrtc/examples',
+  'webrtc/modules/audio_mixer',
   'webrtc/modules/bitrate_controller',
   'webrtc/modules/congestion_controller',
   'webrtc/modules/pacing',

webrtc/modules/audio_mixer/source/new_audio_conference_mixer_impl.h

@@ -14,6 +14,7 @@
 #include <list>
 #include <map>
 #include <memory>
+#include <vector>
 
 #include "webrtc/base/thread_checker.h"
 #include "webrtc/engine_configurations.h"