Exporting document to .xhtml produces AppArmor deny log entry. Add
AppArmor rule for accessing /etc/xml/catalog as it is needed by libxml2.

Change-Id: If62046059c4e1d458cca895f14f7d1e3f537c527
Reviewed-on: https://gerrit.libreoffice.org/59005
Tested-by: Jenkins
Reviewed-by: Rene Engelhard <rene@debian.org>