Specify sddm package as requested by build error

nixos/desktop.nix

@@ -17,6 +17,7 @@
   services.xserver.enable = true;
   services.displayManager.sddm = {
     enable = true;
+    package = pkgs.kdePackages.sddm;
     extraPackages = [pkgs.kdePackages.sddm-kcm];
     wayland.enable = true;
     enableHidpi = true;