libiscsiutil: Record dependency on libmd

MFC after: 3 days
Fixes: 6378393308bc ("Add an internal libiscsiutil library.")
Sponsored by: Klara, Inc.
Sponsored by: NetApp, Inc.
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D55596

(cherry picked from commit 05ca4837a3e9a413aabcf005abb14fff35088476)