ACPI: Add missing prototype_for arch_post_acpi_subsys_init()

In commit e7ff3a47 (x86/amd: Check for the C1E bug post ACPI subsystem
init) a new function arch_post_acpi_subsys_init() was introduced. This
weak function can potentially be overridden on a per arch basis, introduce
the prototype for clarity.

Silence the following gcc warning (W=1):

  init/main.c:484:20: warning: no previous prototype for ‘arch_post_acpi_subsys_init’ [-Wmissing-prototypes]

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>