base/Dockerfile: Add missing libffi-dev dependency for swh-indexer
See https://jenkins.softwareheritage.org/job/DCIDX/job/tox/629/console
Migrated from D2143 (view on Phabricator)
See https://jenkins.softwareheritage.org/job/DCIDX/job/tox/629/console
Migrated from D2143 (view on Phabricator)