fix iptables build

git-svn-id: svn://svn.openwrt.org/openwrt/branches/buildroot-ng/openwrt@3867 3c298f89-4303-0410-b956-a3cf2f4a3e73

package/iptables/Makefile

 # $Id$
 
 include $(TOPDIR)/rules.mk
+include $(TOPDIR)/package/kernel.mk
 
 PKG_NAME:=iptables
 PKG_VERSION:=1.3.5