ipv6: include <net/addrconf.h> for missing declarations

Include <net/addrconf.h> for the missing declarations of
various functions. Fixes the following sparse warnings:

net/ipv6/addrconf_core.c:94:5: warning: symbol 'register_inet6addr_notifier' was not declared. Should it be static?
net/ipv6/addrconf_core.c:100:5: warning: symbol 'unregister_inet6addr_notifier' was not declared. Should it be static?
net/ipv6/addrconf_core.c:106:5: warning: symbol 'inet6addr_notifier_call_chain' was not declared. Should it be static?
net/ipv6/addrconf_core.c:112:5: warning: symbol 'register_inet6addr_validator_notifier' was not declared. Should it be static?
net/ipv6/addrconf_core.c:118:5: warning: symbol 'unregister_inet6addr_validator_notifier' was not declared. Should it be static?
net/ipv6/addrconf_core.c:125:5: warning: symbol 'inet6addr_validator_notifier_call_chain' was not declared. Should it be static?
net/ipv6/addrconf_core.c:237:6: warning: symbol 'in6_dev_finish_destroy' was not declared. Should it be static?

Signed-off-by: Ben Dooks (Codethink) <ben.dooks@codethink.co.uk>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>