dax: fix false CONFIG_BLOCK dependency

In the BLOCK=n case the dax core does not need to / must not emit the
block-device-dax helpers. Otherwise it leads to compile errors.

Cc: Arnd Bergmann <arnd@arndb.de>
Reported-by: Fabian Frederick <fabf@skynet.be>
Fixes: ef510424 ("block, dax: move 'select DAX' from BLOCK to FS_DAX")
Signed-off-by: Dan Williams <dan.j.williams@intel.com>