ipv6: remove duplicated 'net/lwtunnel.h' include

Remove all but the first include of net/lwtunnel.h from seg6_iptunnel.c.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Lv Ruyi <lv.ruyi@zte.com.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>