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