nixpkgs: add xrdb

.config/nixpkgs/overlays/userPackages.nix

@@ -57,6 +57,7 @@ in
       ;
 
 
+    xrdb = self.xorg.xrdb;
     qterminal = self.lxqt.qterminal;
     mps-youtube = self.python36Packages.mps-youtube;
     keepassxc = nixpkgs-unstable.keepassxc;