Export scandir_b from libc.

Apparently it was not exported, because scandir_b.c was not included
into libc SRCS. Export it with the CURRENT-13 version.

Also, because it was not exported before ino64, clean up
scandir-compat11.c.

PR: 248572
Reported by: Alex S <iwtcex@gmail.com>
Reviewed by: emaste
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D26020