cast arg to the right type

According to desc., typelib_TypeDescription can be safely used where
typelib_TypeDescriptionReference is expected.

Change-Id: Ibc3effd7e2894ba6ff738503e747f5157c146454
(cherry picked from commit ef8b94ab0e689851871f9783d4b1f3724177cf3a)
Reviewed-on: https://gerrit.libreoffice.org/11177
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>