headers, xtables: Add missing #include <linux/netfilter.h>

Various headers use union nf_inet_addr, defined in <linux/netfilter.h>.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Acked-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>