net: Remove leftover include from nftables.h

Commit db3685b4

 ("net: remove obsolete members from struct net")
removed the uses of struct list_head from this header, without removing
the corresponding included header.

Signed-off-by: Jörn-Thorben Hinz <jthinz@mailbox.tu-berlin.de>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>