ACPI: APEI build fix

as GHES is optional...

When # CONFIG_ACPI_APEI_GHES is not set:

(.init.text+0x4c22): undefined reference to `ghes_disable'

Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Len Brown <len.brown@intel.com>