ACPI: APEI: Silence missing prototype warnings

Silence the following warnings when make W=1:

 | CC   drivers/acpi/apei/apei-base.c
 |      warning: no previous prototype for 'arch_apei_enable_cmcff' [-Wmissing-prototypes]
 |              int __weak arch_apei_enable_cmcff(struct acpi_hest_header *hest_hdr,
 |                         ^
 | CC   drivers/acpi/apei/apei-base.c
 |      warning: no previous prototype for 'arch_apei_report_mem_error' [-Wmissing-prototypes]
 |              void __weak arch_apei_report_mem_error(int sev,
 |                          ^

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>