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

Change-Id: Ib66134bd4072dfe0ce3bc36aa684cee710921235
Reviewed-on: https://gerrit.libreoffice.org/45868Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>