Change device-mapper's DAX dependency to require the presence of at
least one DAX_DRIVER. This allows device-mapper to be built without
bringing the DAX core along which is especially wasteful when there are
no DAX drivers, like BLK_DEV_PMEM, configured.

Cc: Alasdair Kergon <agk@redhat.com>
Reported-by: NBart Van Assche <Bart.VanAssche@wdc.com>
Reported-by: Nkbuild test robot <lkp@intel.com>
Reported-by: NArnd Bergmann <arnd@arndb.de>
Reviewed-by: NMike Snitzer <snitzer@redhat.com>
Signed-off-by: NDan Williams <dan.j.williams@intel.com>