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

Remove all but the first include of net/lwtunnel.h from seg6_iptunnel.c.
Reported-by: NZeal Robot <zealci@zte.com.cn>
Signed-off-by: NLv Ruyi <lv.ruyi@zte.com.cn>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/seg6_iptunnel.c

@@ -26,7 +26,6 @@
 #ifdef CONFIG_IPV6_SEG6_HMAC
 #include <net/seg6_hmac.h>
 #endif
-#include <net/lwtunnel.h>
 #include <linux/netfilter.h>
 
 static size_t seg6_lwt_headroom(struct seg6_iptunnel_encap *tuninfo)