md5: Include <osreldate.h> for __FreeBSD_version.

It is usually provided by <sys/param.h>, but not when bootstrapping.

Fixes: 4849767cb16a
MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: yuripv, kevans
Differential Revision: https://reviews.freebsd.org/D40018