Fix buildbot breakage by disabling libidn2 that has been recently
added to the packages.

lftp has apparently switched from libidn to libidn2 some time ago,
so the old configure directive was actually wrong, as can be seen
from build log.

  > configure: WARNING: unrecognized options: --without-libidn
  > ...
  > Package lftp is missing dependencies for the following libraries:
  > libidn2.so.0

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>