TT#14008 fix compiler warning from libavformat

Change-Id: Ia22a0c66c511973e935d5a4e40a744d8f92eb19e

recording-daemon/mix.c

@@ -92,7 +92,7 @@ int mix_config(mix_t *mix, const format_t *format) {
 
 	// amix
 	err = "no amix filter available";
-	AVFilter *flt = avfilter_get_by_name("amix");
+	const AVFilter *flt = avfilter_get_by_name("amix");
 	if (!flt)
 		goto err;