And include <cstddef> where necessary.

Change-Id: Icc1208528d6a8b04375d55ccbf3cd6ef046b454f
Reviewed-on: https://gerrit.libreoffice.org/23796Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>