glusterfs: Enable parallel building.

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

Changed files
+2
pkgs
tools
filesystems
glusterfs
+2
pkgs/tools/filesystems/glusterfs/default.nix
···
makeFlags = "DESTDIR=$(out)";
+
enableParallelBuilding = true;
+
postInstall = ''
cp -r $out/$out/* $out
rm -r $out/nix