Silence an annoying message emitted for W=1 builds:

drivers/acpi/x86/apple.c:30:6: warning: no previous declaration for 'acpi_extract_apple_properties' [-Wmissing-declarations]
Reported-by: Nkernel test robot <lkp@intel.com>
Signed-off-by: NLukas Wunner <lukas@wunner.de>
Reviewed-by: NAndy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>