[audio] Quieten jack about xruns

They messages are annoying at the moment, but I will want them when I
get around to chasing down the root cause, so hidden behind a developer
mask.

libs/audio/renderer/snd_jack.c

@@ -306,7 +306,9 @@ snd_jack_error (const char *desc)
 static int
 snd_jack_xrun (void *arg)
 {
+	if (developer->int_val & SYS_SND) {
 		fprintf (stderr, "snd_jack: xrun\n");
+	}
 	return 0;
 }