mkosi: install libidn2 in the final system (#6173)

mkosi: install libidn2 in the final system (#6173)

Recent systemd compiled in a build root which had libidn2-devel installed would
not run in the default installation root which had just libidn and did not have
libidn2. Since a8a2a0ed64f727c82e libidn2-devel (or equivalent) is installed in
the build root. Normally, since we install systemd in the installation root,
all dependencies that are needed for the compiled systemd to run are present,
but libidn2 is a new dependency, so the distro packages would just pull in
libidn, and not libidn2.

Temporily add libdn2 to explicitly specified Packages. We can remove that once
distro-compiled systemds have moved to libind2.