das_watchdog: fix service type

nixos/modules/services/monitoring/das_watchdog.nix

···
       wantedBy = [ "multi-user.target" ];
       serviceConfig = {
         User = "root";
-        Type = "oneshot";
+        Type = "simple";
         ExecStart = "${das_watchdog}/bin/das_watchdog";
         RemainAfterExit = true;
       };