cf. <https://gerrit.libreoffice.org/#/c/16357> "Revert the removal of cast to
build on old poppler"

Change-Id: I75184cec64b4f32e0cf9adedeb84c8591be4fc73
(cherry picked from commit b51ba9e3)
Reviewed-on: https://gerrit.libreoffice.org/16643Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>