iptables: Makefile: only build ip6tc, if IPv6 is enabled

when disabling ipv6, the iptables build breaks without a manul clean or this patch

Signed-off-by: Bastian Bittorf <bittorf@bluebottle.com>

SVN-Revision: 40916

package/network/utils/iptables/Makefile

@@ -316,7 +316,7 @@ define Package/libiptc
 $(call Package/iptables/Default)
   SECTION:=libs
   CATEGORY:=Libraries
-  DEPENDS:=+libip4tc +libip6tc
+  DEPENDS:=+libip4tc +IPV6:libip6tc
   TITLE:=IPv4/IPv6 firewall - shared libiptc library (compatibility stub)
 endef