Quiet pod2html warnings

--quiet stops warnings of this sort:

    Cannot find "BIO_read_ex" in podpath: cannot find suitable replacement path, cannot resolve link

We know what causes these warnings, it's perfectly innocuous, and we
don't want to hear it any more.

Partial fix for #3254

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6304)