ACPI / NUMA: Add stub function for pxm_to_node()

After commit 01feba59 ("ACPI: Do not create new NUMA domains from
ACPI static tables that are not SRAT"):

$ scripts/config --file arch/x86/configs/x86_64_defconfig -d NUMA -e ACPI_NFIT

$ make -skj"$(nproc)" distclean defconfig drivers/acpi/nfit/
drivers/acpi/nfit/core.c: In function ‘acpi_nfit_register_region’:
drivers/acpi/nfit/core.c:3010:27: error: implicit declaration of
function ‘pxm_to_node’; did you mean ‘xa_to_node’?
[-Werror=implicit-function-declaration]
 3010 |   ndr_desc->target_node = pxm_to_node(spa->proximity_domain);
      |                           ^~~~~~~~~~~
      |                           xa_to_node
cc1: some warnings being treated as errors
...

Add a stub function like acpi_map_pxm_to_node() had so that the build
continues to work.

Fixes: 01feba59 ("ACPI: Do not create new NUMA domains from ACPI static tables that are not SRAT")
Signed-off-by: NNathan Chancellor <natechancellor@gmail.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Acked-by: NJonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: NHanjun Guo <guohanjun@huawei.com>
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>

include/acpi/acpi_numa.h

@@ -22,5 +22,10 @@ extern int acpi_numa __initdata;
 extern void bad_srat(void);
 extern int srat_disabled(void);
 
+#else				/* CONFIG_ACPI_NUMA */
+static inline int pxm_to_node(int pxm)
+{
+	return 0;
+}
 #endif				/* CONFIG_ACPI_NUMA */
 #endif				/* __ACP_NUMA_H */