mail/dbmail: unbreak build after a315965d7d62ea69f5aed95c37e7ad36c900dc61 (+)

Restore version for libzdb.so.19 dependency checks to avoid name conflict
with base libzdb.so from recent ZFS presented at least on 15.x and -current

% pkg which /usr/lib/libzdb.so
/usr/lib/libzdb.so was installed by package FreeBSD-zfs-dev-16