Fix clang warnings in AudioFlinger

Change-Id: I0fa61025c979709ad7d655bc717df5f194b6089e
diff --git a/include/media/AudioTrack.h b/include/media/AudioTrack.h
index 3a60217..5a50280 100644
--- a/include/media/AudioTrack.h
+++ b/include/media/AudioTrack.h
@@ -27,7 +27,7 @@
 
 // ----------------------------------------------------------------------------
 
-class audio_track_cblk_t;
+struct audio_track_cblk_t;
 class AudioTrackClientProxy;
 class StaticAudioTrackClientProxy;