l10ntools/inc/helper.hxx: bin unused header <cassert>

Change-Id: Ib2c43d9b158423c023a5bfd4da324554aa04902e
Reviewed-on: https://gerrit.libreoffice.org/11316
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>
Tested-by: Norbert Thiebaud <nthiebaud@gmail.com>