nixos: use functionTo to prevent evaluation errors while merging

Without this patch merging options like
services.xserver.windowManager.xmonad.extraPackages
results in the evaluation error:

  error: value is a list while a set was expected, at nixpkgs/lib/options.nix:77:23

With this patch we get the desired merging behaviour that just concatenates the
resulting package lists.

(cherry picked from commit 6e99f9fdecb1f28308c8e0aed0fc851737354864)

Co-Authored-By: Silvan Mosberger <contact@infinisil.com>

nixos/modules/services/misc/gitit.nix

@@ -42,6 +42,7 @@ let
       };
 
       extraPackages = mkOption {
+        type = types.functionTo (types.listOf types.package);
         default = self: [];
         example = literalExample ''
           haskellPackages: [