network-manager-l2tp: add homepage

pkgs/tools/networking/network-manager/l2tp.nix

···
   meta = with stdenv.lib; {
     description = "L2TP plugin for NetworkManager";
     inherit (networkmanager.meta) platforms;
+    homepage = https://github.com/seriyps/NetworkManager-l2tp;
     license = licenses.gpl2;
     maintainers = with maintainers; [ abbradar ];
   };