mkosi: update libqrencode in Debian config

In Debian unstable package libqrencode-dev is version 4.0.2-1, and the
corresponding runtime library is provided by package libqrencode4.

This change fixes the following error when running journalctl:

    root@image:~# journalctl
    journalctl: error while loading shared libraries: libqrencode.so.4: cannot
    open shared object file: No such file or directory

This change also fixes the following boot failures in
systemd-journal-flush.service and systemd-journal-catalog-update.service:

    [FAILED] Failed to start Flush Journal to Persistent Storage.
    [FAILED] Failed to start Rebuild Journal Catalog.

See also #4949