The page header and footer panels were completely missing labelling

The "Custom" label is actually used a a resource, suppress the labelling
warning for it.

Change-Id: I3ca6b1984fab23478a11e916f26702592a5ee991
Reviewed-on: https://gerrit.libreoffice.org/52948Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>