gpart: Broken on Darwin

Never had a successful Darwin build.

http://hydra.nixos.org/build/38306346

pkgs/tools/filesystems/gpart/default.nix

···
       or device.
     '';
     license = licenses.gpl2Plus;
-    platforms = platforms.unix;
     maintainers = with maintainers; [ nckx ];
+    platforms = platforms.linux;
   };
 }