nixos/networkd: Fix eval error for defaultGateway

Regression introduced by 0cb487ee04.

This changed the result for defaultGateway to be a submodule instead of
just a plain string, so instead of using just cfg.defaultGateway we need
to pass cfg.defaultGateway.address now.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @abbradar

nixos/modules/tasks/network-interfaces-systemd.nix

@@ -62,9 +62,9 @@ in
         genericNetwork = override: {
           DHCP = override (dhcpStr cfg.useDHCP);
         } // optionalAttrs (cfg.defaultGateway != null) {
-          gateway = override [ cfg.defaultGateway ];
+          gateway = override [ cfg.defaultGateway.address ];
         } // optionalAttrs (cfg.defaultGateway6 != null) {
-          gateway = override [ cfg.defaultGateway6 ];
+          gateway = override [ cfg.defaultGateway6.address ];
         } // optionalAttrs (domains != [ ]) {
           domains = override domains;
         };