ACPI: Add missing prototype_for arch_post_acpi_subsys_init()

In commit e7ff3a4 (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>

include/linux/acpi.h

@@ -578,6 +578,7 @@ int acpi_match_platform_list(const struct acpi_platform_list *plat);
 
 extern void acpi_early_init(void);
 extern void acpi_subsystem_init(void);
+extern void arch_post_acpi_subsys_init(void);
 
 extern int acpi_nvs_register(__u64 start, __u64 size);