Reinstate libwmf0.2-7 as a transitional package

This avoids having to do a transition with lockstep upgrades for affected packages, and should also make upgrades from Debian 10 to Debian 11 more straightforward.

See https://lists.debian.org/debian-release/2020/08/msg00253.html and https://lists.debian.org/debian-release/2020/08/msg00255.html for more information.