(as std::mem_fun is gone by default at least from recent libc++ in C++17 mode)

Change-Id: I109dd7e04ed2914eb8768bb8139758b5ad60dadc
Reviewed-on: https://gerrit.libreoffice.org/45862Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>