make the implementation use the same type than the declaration

Change-Id: I860e6db6ea4a7a7194283ba4fc23ae30cb1de4f8
Reviewed-on: https://gerrit.libreoffice.org/8201
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>
Tested-by: Norbert Thiebaud <nthiebaud@gmail.com>