nixos/sddm: add package option

nixos/modules/services/x11/display-managers/sddm.nix

@@ -7,7 +7,7 @@ let
   cfg = dmcfg.sddm;
   xEnv = config.systemd.services.display-manager.environment;
 
-  sddm = pkgs.libsForQt5.sddm;
+  sddm = cfg.package;
 
   iniFmt = pkgs.formats.ini { };
 
@@ -108,6 +108,8 @@ in
         '';
       };
 
+      package = mkPackageOption pkgs [ "plasma5Packages" "sddm" ] {};
+
       enableHidpi = mkOption {
         type = types.bool;
         default = true;