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

Change-Id: I74a7ff92385068cb4de1865921bd7c2509859b63
Reviewed-on: https://gerrit.libreoffice.org/45859Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>