borgbackup: Add acl library to build inputs on linux platform

The acl libraray is only required by the borgbackup package if building on a
linux platform. Adding it only in this case should be fine. Also see the
conditional in the setup.py at [1].

[1] https://github.com/borgbackup/borg/blob/72232a9bd573aeb4818b36c9c8764b6008e9283e/setup.py#L768