dax/hmem: build hmem device support as module if possible

dax/hmem: build hmem device support as module if possible

When device_hmem.o is enabled but dax itself is a loadable module, the
dax_hmem support fails to link because Kbuild never compiles built-in
code under drivers/dax:

ERROR: modpost: "walk_hmem_resources" [drivers/dax/hmem/dax_hmem.ko] undefined!

Make sure that drivers/dax is entered for compiling built-in code
even with CONFIG_DAX=m.

Fixes: 7dab174e2e27 ("dax/hmem: Move hmem device registration to dax_hmem.ko")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20230214131913.1431969-1-arnd@kernel.org
Signed-off-by: Dan Williams <dan.j.williams@intel.com>