nixos/fwupd: add polkit dependency

We are building fwupd daemon with polkit support which means
polkit daemon is required.

Previously polkit was enabled by default via udisks2 but that
stopped with f763710065
breaking the fwupd installed tests as a result.

Let’s add the polkit dependency to the fwupd module to ensure polkit is available.

nixos/modules/services/hardware/fwupd.nix

@@ -126,6 +126,8 @@ in {
     services.udev.packages = [ cfg.package ];
 
     systemd.packages = [ cfg.package ];
+
+    security.polkit.enable = true;
   };
 
   meta = {