ghostunnel: conditional systemd

Make the systemd-specific configuration of `ghostunnel`s service (see
portability](https://github.com/NixOS/nixpkgs/blob/master/nixos/doc/manual/development/modular-services.md#portability-modular-service-portability).

The idea here is that making the module easier to use in systems not
using systemd could make for a best practice in writing such modules.
As `ghostunnel` is the first such portable module, it seems all the more
relevant to apply best practices there for when it is used as a source
of inspiration in portable modules for further applications.

pkgs/by-name/gh/ghostunnel/service.nix

@@ -220,7 +220,8 @@ in
         ++ optional cfg.unsafeTarget "--unsafe-target"
         ++ cfg.extraArguments;
     };
-
+  }
+  // lib.optionalAttrs (options ? systemd) {
     # refine the service
     systemd.service = {
       after = [ "network.target" ];