Since libmd was added to the bootstrap, building main on stable/14
fails because of an incompatibility in its old md4.h. Fix this by
always including md4.h in the bootstrap headers, instead of only
doing so when building on a non-FreeBSD host.
Fixes: 50de0bf50512 ("flua: Always build as a bootstrap tool")
Reported by: olce
Sponsored by: https://www.patreon.com/bsdivy