ACPI / numa: Make __acpi_map_pxm_to_node(), acpi_get_pxm() static

__acpi_map_pxm_to_node() and acpi_get_pxm() are only used within
drivers/acpi/numa.c.  This makes them static and removes their
declarations.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>