xorg.xf86videoxgi: fix build with upstream patch

pkgs/servers/x11/xorg/overrides.nix

···
         url = "https://cgit.freedesktop.org/xorg/driver/xf86-video-xgi/patch/?id=bd94c475035739b42294477cff108e0c5f15ef67";
         sha256 = "0myfry07655adhrpypa9rqigd6rfx57pqagcwibxw7ab3wjay9f6";
       })
+      (args.fetchpatch {
+        url = "https://cgit.freedesktop.org/xorg/driver/xf86-video-xgi/patch/?id=78d1138dd6e214a200ca66fa9e439ee3c9270ec8";
+        sha256 = "0z3643afgrync280zrp531ija0hqxc5mrwjif9nh9lcnzgnz2d6d";
+      })
     ];
   };