igmpproxy: remove firewall rules when service is stopped

Remove multicast routing firewall rules when the igmpproxy is stopped by
triggering a firewall config change.
Keeping the firewall open from the wan for igmp and udp multicast is not
desired when the igmpproxy service is inactive.

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>

package/network/services/igmpproxy/files/igmpproxy.init

@@ -132,3 +132,7 @@ start_service() {
 service_started() {
 	procd_set_config_changed firewall
 }
+
+stop_service() {
+	procd_set_config_changed firewall
+}