res_ari_events: Fix memory leak in mustache template.

ASTERISK-25308 fixed a memory leak in res_ari_events.c, but
this file is regenerated by a template and the template was
not fixed.

Change-Id: Ied4c6deae89d21f87f9cf99676b1d055aa83b38b

rest-api-templates/res_ari_resource.c.mustache

@@ -291,6 +291,7 @@ static int unload_module(void)
 {{#has_websocket}}
 	ao2_cleanup({{full_name}}.ws_server);
 	{{full_name}}.ws_server = NULL;
+	ast_ari_websocket_events_event_websocket_dtor();
 {{/has_websocket}}
 {{/apis}}
 	stasis_app_unref();