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>

include/linux/acpi.h

@@ -1491,6 +1491,10 @@ static inline void acpi_init_pcc(void) { }
 
 #ifdef CONFIG_ACPI_FFH
 void acpi_init_ffh(void);
+extern int acpi_ffh_address_space_arch_setup(void *handler_ctxt,
+					     void **region_ctxt);
+extern int acpi_ffh_address_space_arch_handler(acpi_integer *value,
+					       void *region_context);
 #else
 static inline void acpi_init_ffh(void) { }
 #endif