borgbackup: Add llfuse to propagated build inputs if not on darwin

The llfuse package depends on fuse which refuses to build on darwin. But
according to a comment in the setup.py of borgbackup [1] it's ok to leave it out
if it's not available. Most of borgbackup should work without it. Would be great
to make it work on darwin but i am not sure if it's possible to get fuse to work
on darwin. I do not know enough about it ;)

After this modification at least the "borg mount" subcommand is broken due to
the missing llfuse module. But the rest seems to work normally.

[1] https://github.com/borgbackup/borg/blob/72232a9bd573aeb4818b36c9c8764b6008e9283e/setup.py#L32