freebsd: fix missing headers in distribution tarball

arc_os.h and freebsd_event.h aren't included in release tarballs, so the
build fails on FreeBSD. This fixes it.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #15963