ipv6: remove unused in6_addr struct

struct in6_addr isn't used anymore in inet6_connection_sock.h, removing
the forward declaration.

Fixes: 1b33bc3e ("ipv6: remove obsolete inet6 functions")
Signed-off-by: NLuis de Bethencourt <luisbg@osg.samsung.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/inet6_connection_sock.h

@@ -15,7 +15,6 @@
 
 #include <linux/types.h>
 
-struct in6_addr;
 struct inet_bind_bucket;
 struct request_sock;
 struct sk_buff;