ACPI: fix CONFIG_NET=n acpi_bus_generate_netlink_event build failure

drivers/acpi/event.c:243: error: 'acpi_generate_netlink_event' undeclared
here (not in a function)

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Len Brown <len.brown@intel.com>