libobs: Mark parameter as constant if not modifying

libobs/obs-source.c

@@ -1050,7 +1050,7 @@ static void handle_ts_jump(obs_source_t *source, uint64_t expected,
 }
 
 static void source_signal_audio_data(obs_source_t *source,
-		struct audio_data *in, bool muted)
+		const struct audio_data *in, bool muted)
 {
 	pthread_mutex_lock(&source->audio_cb_mutex);