ACPI: NUMA: add missing include in acpi_numa.h

Right now if a file includes acpi_numa.h and they don't happen to include
linux/numa.h before it, they get the following warning:

./include/acpi/acpi_numa.h:9:5: warning: "MAX_NUMNODES" is not defined [-Wundef]
 #if MAX_NUMNODES > 256
     ^~~~~~~~~~~~

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>