mtpfs: mark as broken on darwin

pkgs/tools/filesystems/mtpfs/default.nix

···
     platforms = platforms.all;
     license = licenses.gpl3;
     maintainers = [ maintainers.qknight ];
+    broken = stdenv.isDarwin; # never built on Hydra https://hydra.nixos.org/job/nixpkgs/trunk/mtpfs.x86_64-darwin
   };
 }