automon: restore mixing of the both channels after recording stops

This is a regression over Asterisk 11, introduced by
2dc8a06006. Previously, recordings started via
the automon DTMF code would automatically be mixed together using sox because
app_monitor would be called with the m option. This commit restores this
behavior.

Change-Id: Ibaf58684285c3f1b6ca3714524e6d638ae3b3759

bridges/bridge_builtin_features.c

@@ -200,6 +200,8 @@ static void start_automonitor(struct ast_bridge_channel *bridge_channel, struct
 		return;
 	}
 
+	ast_monitor_setjoinfiles(peer_chan, 1);
+
 	if (features_cfg && !ast_strlen_zero(features_cfg->courtesytone)) {
 		ast_bridge_channel_queue_playfile(bridge_channel, NULL, features_cfg->courtesytone, NULL);
 		ast_bridge_channel_write_playfile(bridge_channel, NULL, features_cfg->courtesytone, NULL);