ipv6: add include file to suppress sparse warnings

commit f88c91dd

 ("ipv6: statically link
register_inet6addr_notifier()" added following sparse warnings :

net/ipv6/addrconf_core.c:83:5: warning: symbol
'register_inet6addr_notifier' was not declared. Should it be static?
net/ipv6/addrconf_core.c:89:5: warning: symbol
'unregister_inet6addr_notifier' was not declared. Should it be static?
net/ipv6/addrconf_core.c:95:5: warning: symbol
'inet6addr_notifier_call_chain' was not declared. Should it be static?

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>