ACPI: processor_idle: Silence missing prototype warnings

Silence the following warnings when built with W=1:

 | CC   drivers/acpi/processor_idle.c
 |      warning: no previous prototype for 'acpi_processor_ffh_lpi_probe' [-Wmissing-prototypes]
 |		int __weak acpi_processor_ffh_lpi_probe(unsigned int cpu)
 |                         ^
 | CC   drivers/acpi/processor_idle.c
 |      warning: no previous prototype for 'acpi_processor_ffh_lpi_enter' [-Wmissing-prototypes]
 |              int __weak acpi_processor_ffh_lpi_enter(struct acpi_lpi_state *lpi)
 |                         ^

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>