kmscube: Fix src

Someone sedded a bit too eagerly.

pkgs/os-specific/linux/kmscube/default.nix

···
   name = "kmscube-2017-03-19";
 
   src = fetchgit {
-    url = git://anongit.freedesktop.org/libGLU_combined/kmscube;
+    url = git://anongit.freedesktop.org/mesa/kmscube;
     rev = "b88a44d95eceaeebc5b9c6972ffcbfe9eca00aea";
     sha256 = "029ccslfavz6jllqv980sr6mj9bdbr0kx7bi21ra0q9yl2vh0yca";
   };