Avoid file-rc being considered for removal by deborphan

For whatever reason deborphan seems to consider file-rc
being available for removal since a few days:

| =====   shell: DEBORPHAN/98-clean-chroot   =====
| [...]
| (Reading database ... 24676 files and directories currently installed.)
| Removing file-rc ...
| Purging configuration files for file-rc ...
| Please purge /var/backups/rc.* manually.
| DEBORPHAN/98-clean-chroot OK.