home: x: add 'xsel' package

I like it better than 'xclip'.

home/x/default.nix

@@ -1,4 +1,4 @@
-{ config, lib, ... }:
+{ config, lib, pkgs, ... }:
 let
   cfg = config.my.home.x;
 in
@@ -14,5 +14,9 @@ in
 
   config = lib.mkIf cfg.enable {
     xsession.enable = true;
+
+    home.packages = with pkgs; [
+      xsel
+    ];
   };
 }