ACPI: processor: Silence missing prototype warnings

Silence the following warnings when built with W=1:

 | CC   drivers/acpi/acpi_processor.c
 |      warning: no previous prototype for 'arch_register_cpu' [-Wmissing-prototypes]
 |              int __weak arch_register_cpu(int cpu)
 |                         ^
 | CC   drivers/acpi/acpi_processor.c
 |      warning: no previous prototype for 'arch_unregister_cpu' [-Wmissing-prototypes]
 |              void __weak arch_unregister_cpu(int cpu) {}
 |                          ^

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>