avformat/tcp: Put struct sockaddr_in6 under #if

Fixes: error: dereferencing pointer to incomplete type

Tested-by: Dave Yeo <daveryeo@telus.net>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/tcp.c

@@ -122,6 +122,7 @@ static int tcp_open(URLContext *h, const char *uri, int flags)
     cur_ai = ai;
 
  restart:
+#if HAVE_STRUCT_SOCKADDR_IN6
     // workaround for IOS9 getaddrinfo in IPv6 only network use hardcode IPv4 address can not resolve port number.
     if (cur_ai->ai_family == AF_INET6){
         struct sockaddr_in6 * sockaddr_v6 = (struct sockaddr_in6 *)cur_ai->ai_addr;
@@ -129,6 +130,7 @@ static int tcp_open(URLContext *h, const char *uri, int flags)
             sockaddr_v6->sin6_port = htons(port);
         }
     }
+#endif
 
     fd = ff_socket(cur_ai->ai_family,
                    cur_ai->ai_socktype,