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>

include/linux/icmpv6.h

@@ -16,9 +16,9 @@ static inline struct icmp6hdr *icmp6_hdr(const struct sk_buff *skb)
 
 typedef void ip6_icmp_send_t(struct sk_buff *skb, u8 type, u8 code, __u32 info,
 			     const struct in6_addr *force_saddr);
-#if IS_BUILTIN(CONFIG_IPV6)
 void icmp6_send(struct sk_buff *skb, u8 type, u8 code, __u32 info,
 		const struct in6_addr *force_saddr);
+#if IS_BUILTIN(CONFIG_IPV6)
 static inline void icmpv6_send(struct sk_buff *skb, u8 type, u8 code, __u32 info)
 {
 	icmp6_send(skb, type, code, info, NULL);