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>