ipv6: silence compilation warning for non-IPV6 builds

The W=1 compilation of allmodconfig generates the following warning:

net/ipv6/icmp.c:448:6: warning: no previous prototype for 'icmp6_send' [-Wmissing-prototypes]
  448 | void icmp6_send(struct sk_buff *skb, u8 type, u8 code, __u32 info,
      |      ^~~~~~~~~~

Fix it by providing function declaration for builds with ipv6 as a module.

Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>