gltron: mark as broken

(cherry picked from commit 23d01bb90c9d6ba74f507fe69439f7e1e0a3f238)
Signed-off-by: Domen Kožar <[e-mail]>

pkgs/games/gltron/default.nix

···
     license = stdenv.lib.licenses.gpl2Plus;
     maintainers = with stdenv.lib.maintainers; [viric];
     platforms = with stdenv.lib.platforms; linux;
+    broken = true; # see http://sourceforge.net/p/gltron/bugs/15
   };
 }