nixosTests.power-profiles-daemon: enable polkit

I think this is required for the gdbus invocations used to implement
the test, rather than for power-profiles-daemon itself.

Fixes: a813be071ce ("nixos/polkit: don't enable by default")

nixos/tests/power-profiles-daemon.nix

···
     maintainers = [ mvnetbiz ];
   };
   nodes.machine = { pkgs, ... }: {
+    security.polkit.enable = true;
     services.power-profiles-daemon.enable = true;
     environment.systemPackages = [ pkgs.glib ];
   };