Fix bootstrapping libmd on older systems after r314709.

This follows another fix to bootstrap libmd after r313404. The
MD5FileChunk prototype is needed to build libmd, but it is
only reliably in the src tree's sys/md5.h header. Rather than
polluting the legacy build with this header for the entire build,
just symlink it in here for now as is done in the elftoolchain
build. Libmd is already referencing other src tree headers by
its used of CFLAGS+= ${SRCTOP}/sys/crypto/sha2. This, and
other uses of CFLAGS+= ${SRCTOP}/sys..., may later change to
be in the legacy mechanism.

Reported by: bde, ian, sjg
Tested by: ian