ACPI: fix ia64 allnoconfig build

drivers/acpi/event.c:238: error: conflicting types for ‘acpi_bus_generate_netlink_event’
include/acpi/acpi_bus.h:324: error: previous declaration of ‘acpi_bus_generate_netlink_event’ was here

Signed-off-by: Len Brown <len.brown@intel.com>