ACPI: FFH: Silence missing prototype warnings

Silence the following warnings when built with W=1:

 | CC   drivers/acpi/acpi_ffh.c
 |      warning: no previous prototype for 'acpi_ffh_address_space_arch_setup' [-Wmissing-prototypes]
 |              int __weak acpi_ffh_address_space_arch_setup(void *handler_ctxt,
 |                         ^
 | CC   drivers/acpi/acpi_ffh.c
 |      warning: no previous prototype for 'acpi_ffh_address_space_arch_handler' [-Wmissing-prototypes]
 |              int __weak acpi_ffh_address_space_arch_handler(acpi_integer *value,
 |                         ^

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>