bin/df: fix build with MOUNT_CHAR_DEVS defined

When resolving conflicts due to the unmergewd commit that removed the
code previously made conditional on MOUNT_CHAR_DEVS, I failed to see
that MOUNT needed to be defined in the Makefile and that the include
of mntopts.h was still required for that case.

Fix the build by adding back the missing definitions to the Makefile.