in preparation of removal of deprecated std::binary_function

Change-Id: Ibbf9e4689301d1eb525ee965c75d07077291a6ac
Reviewed-on: https://gerrit.libreoffice.org/39556Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>