[IPV6]: Fix powerpc allmodconfig build warnings.

Introduced by changeset 95e41e93
("[IPV6]: Make ndisc_flow_init() common for later use.")

Reported by Stephen Rothwell.

In file included from net/ipv6/netfilter/ip6_tables.c:21:
include/linux/icmpv6.h:192: warning: 'struct in6_addr' declared inside parameter list
include/linux/icmpv6.h:192: warning: its scope is only this definition or declaration, which is probably not what you want
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/icmpv6.h

@@ -184,6 +184,7 @@ extern void				icmpv6_param_prob(struct sk_buff *skb,
 							  int code, int pos);
 
 struct flowi;
+struct in6_addr;
 extern void				icmpv6_flow_init(struct sock *sk,
 							 struct flowi *fl,
 							 u8 type,