tcp: fix build error if IPv6 is not enabled

$ make M=net/ipv4
  CC      net/ipv4/route.o
In file included from net/ipv4/route.c:102:0:
include/net/tcp.h: In function ‘tcp_v6_iif’:
include/net/tcp.h:738:32: error: ‘union <anonymous>’ has no member named ‘h6’
  return TCP_SKB_CB(skb)->header.h6.iif;
Signed-off-by: Eric Dumazet <edumazet@google.com>
Fixes: 870c3151 ("ipv6: introduce tcp_v6_iif()")
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -730,6 +730,7 @@ struct tcp_skb_cb {
 #define TCP_SKB_CB(__skb)	((struct tcp_skb_cb *)&((__skb)->cb[0]))
 
 
+#if IS_ENABLED(CONFIG_IPV6)
 /* This is the variant of inet6_iif() that must be used by TCP,
  * as TCP moves IP6CB into a different location in skb->cb[]
  */
@@ -737,6 +738,7 @@ static inline int tcp_v6_iif(const struct sk_buff *skb)
 {
 	return TCP_SKB_CB(skb)->header.h6.iif;
 }
+#endif
 
 /* Due to TSO, an SKB can be composed of multiple actual
  * packets.  To keep these tracked properly, we use this.