da9030_battery: Include notifier.h

randconfig complains about:
drivers/power/da9030_battery.c:113: error: field ‘nb’ has incomplete type

because there is no direct include for notifier.h which defines
struct notifier_block.

Signed-off-by: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>