ACPI: PM: Silence missing prototype warning

Silence the following warning when built with W=1:

 | CC   drivers/acpi/device_pm.c
 |      warning: no previous prototype for function 'acpi_subsys_restore_early' [-Wmissing-prototypes]
 |              int acpi_subsys_restore_early(struct device *dev)
 |                  ^

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>