notify-recv: optionally return event source from notify_socket_prepare()

src/sysupdate/sysupdate-transfer.c

@@ -1062,7 +1062,8 @@ static int run_callout(
                         SD_EVENT_PRIORITY_NORMAL - 5,
                         helper_on_notify,
                         ctx,
-                        &bind_name);
+                        &bind_name,
+                        /* ret_event_source= */ NULL);
         if (r < 0)
                 return log_error_errno(r, "Failed to prepare notify socket: %m");