The page style panel was completely missing labelling.

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

Change-Id: I741ddea4a1608f0d7084dda79af2ab59a6fc5812
Reviewed-on: https://gerrit.libreoffice.org/52947Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>