gpart.8: add entry for xbootldr partition type

Reviewed by: emaste, markj
Sponsored by: The FreeBSD Foundation
Fixes: dab8138e13de ("g_part,mkimg: Add additional GPT partition types")
Differential Revision: https://reviews.freebsd.org/D56969

(cherry picked from commit 54d0ae979d9596399f33dc7d7245866f86ab7f8e)