Fix compilation of uuid-ossp

Fix compilation of uuid-ossp

This module had a dependency on pgcrypto's md5.c that got removed by
b67b57a.  Instead of the code from pgcrypto, this code can just use the
new cryptohash routines for MD5 as a drop-in replacement, so let's just
do this switch.  This has also the merit to simplify a bit the
compilation of uuid-ossp.

This requires --with-uuid to be reproduced, and I have used e2fs as a
way to reproduce the failure, then test this commit.

Per reports from buildfarm members longfin, florican and sifaka.

Discussion: https://postgr.es/m/X9GToVd3QmWeNvj8@paquier.xyz