gpgconf: Also print revision of libksba.

* dirmngr/dirmngr.c (get_revision_from_blurb): Fix detection of empty
string.
(gpgconf_versions): Print ksba revision.

The latest Libksba version support retrieving of the revision (commit)
id. We now use that or print a question mark.

• Signed-off-by: Werner Koch <wk@gnupg.org>