output/Internal: move the Mutex to struct AudioOutputControl

struct AudioOutput should not know or care about multi-threading.

src/output/Control.cxx

@@ -41,8 +41,7 @@ struct notify audio_output_client_notify;
 AudioOutputControl::AudioOutputControl(AudioOutput *_output,
 				       AudioOutputClient &_client)
 	:output(_output), client(_client),
-	 thread(BIND_THIS_METHOD(Task)),
-	 mutex(output->mutex)
+	 thread(BIND_THIS_METHOD(Task))
 {
 }