netfilter: Enable compiling iptables match cluster

This patch adds the userspace and kernelspace for

- match NETFILTER_XT_MATCH_CLUSTER
  This match can be used to deploy gateway and back-end load-sharing clusters.
- target IP_NF_TARGET_CLUSTERIP
  This module allows you to configure a simple cluster of nodes
  that share a certain IP and MAC address
  without an explicit load balancer in front of them.
  Connections are statically distributed between the nodes in this cluster.

This is used i.e. by strongswan-ha.

Signed-off-by: Christian Scheele <cs@embedd.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43174 3c298f89-4303-0410-b956-a3cf2f4a3e73