bin/df: add back linking with getmntops.c from sbin/mount

A last minute clean-up of the Makefile erroneously removed the
definitions required to link against sbin/mount/getmntops.c which
defines the iovec handling functions used for the unmounted device
case (that has been removed from -CURRENT and 13-STABLE).

Put back the definitions to make the build with MOUNT_CHAR_DEVS
defined succeed.