lightdm_gtk_greeter: fix license

pkgs/applications/display-managers/lightdm/gtk-greeter.nix

···
   meta = with lib; {
     homepage = "https://launchpad.net/lightdm-gtk-greeter";
     platforms = platforms.linux;
-    license = licenses.gpl3;
+    license = licenses.gpl3Plus;
     maintainers = with maintainers; [ ];
   };
 }