glusterfs: Enable parallel building.

I checked for determinism with `nix-build --option build-repeat 10`.

pkgs/tools/filesystems/glusterfs/default.nix

···
 
   makeFlags = "DESTDIR=$(out)";
 
+  enableParallelBuilding = true;
+
   postInstall = ''
     cp -r $out/$out/* $out
     rm -r $out/nix