dnsmasq-manager: forget the watch when the callback hits

It removes the source, we shouldn't try to remove it on dispose() then.

(cherry picked from commit 4fe86b0031)

src/dnsmasq-manager/nm-dnsmasq-manager.c

@@ -203,6 +203,7 @@ dm_watch_cb (GPid pid, gint status, gpointer user_data)
 	}
   
 	priv->pid = 0;
+	priv->dm_watch_id = 0;
 
 	g_signal_emit (manager, signals[STATE_CHANGED], 0, NM_DNSMASQ_STATUS_DEAD);
 }