archivers/py-borgbackup: unbreak on FreeBSD 13

import acl_extended_file_np.c from src/lib/libc/posix1e,
taken from FreeBSD src branch as of 559a218c9b25, 2023-11-01T16:43:37Z
and tell Cython to include it on FreeBSD before 1400033,
i. e., before FreeBSD's libc had it.

This fixes https://github.com/borgbackup/borg/issues/8269 and
makes it unnecessary to look into
PR: 280453