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: 7dab174 ("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>

Author: arndb

drivers/Makefile

@@ -71,7 +71,7 @@ obj-$(CONFIG_FB_INTEL)          += video/fbdev/intelfb/
 obj-$(CONFIG_PARPORT)		+= parport/
 obj-y				+= base/ block/ misc/ mfd/ nfc/
 obj-$(CONFIG_LIBNVDIMM)		+= nvdimm/
-obj-$(CONFIG_DAX)		+= dax/
+obj-y				+= dax/
 obj-$(CONFIG_DMA_SHARED_BUFFER) += dma-buf/
 obj-$(CONFIG_NUBUS)		+= nubus/
 obj-y				+= cxl/