hidpi: Forgotten rounding.

Change-Id: Ie790b40ac86708768f52084538aa08f99df7def6
Reviewed-on: https://gerrit.libreoffice.org/8895
Reviewed-by: Keith Curtis <keithcu@gmail.com>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>