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>