Mark calibre CVE-2018-7889 in wheezy

There is no known fix for this, and a true fix is not possible
without changing the configuration file formats not to allow
executable code.

See:
* https://lists.debian.org/debian-lts/2018/04/msg00098.html
* https://lists.debian.org/debian-lts/2018/05/msg00009.html