firewall3: update to git head

	- allows building without IPv6 support
	- uses more robust rules to cope with missing libext.a
	- uses better linking strategy to avoid symbol clashes with older iptables
	- introduces source compatiblity layer for different libxtables versions

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@36736 3c298f89-4303-0410-b956-a3cf2f4a3e73