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 https://github.com/NixOS/nixpkgs/commit/f7637100655127dffbe14011b45be6a71f0e203d
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 = {