Fix unused-parameter warning in systemd_watchdog_callback

src/or/main.c

@@ -1776,6 +1776,8 @@ static periodic_timer_t *systemd_watchdog_timer = NULL;
 static void
 systemd_watchdog_callback(periodic_timer_t *timer, void *arg)
 {
+  (void)timer;
+  (void)arg;
   sd_notify(0, "WATCHDOG=1");
 }
 #endif