net: include/linux/icmpv6: includecheck fix for icmpv6.h

fix the following 'make includecheck' warning:

  include/linux/icmpv6.h: linux/skbuff.h is included more than once.
Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/icmpv6.h

@@ -171,8 +171,6 @@ struct icmp6_filter {
 #ifdef __KERNEL__
 
 #include <linux/netdevice.h>
-#include <linux/skbuff.h>
-
 
 extern void				icmpv6_send(struct sk_buff *skb,
 						    u8 type, u8 code,