iptables: use correct variable for configure options

Introduced in r23409.

package/iptables/iptables.mk

@@ -7,7 +7,7 @@ IPTABLES_VERSION = 1.4.2
 IPTABLES_SOURCE = iptables-$(IPTABLES_VERSION).tar.bz2
 IPTABLES_SITE = http://ftp.netfilter.org/pub/iptables
 
-IPTABLES_CONF_OPT = --libexecdir=/usr/lib
+IPTABLES_CONF_OPT = --libexecdir=/usr/lib --with-kernel=$(LINUX_HEADERS_DIR)
 ifneq ($(BR2_INET_IPV6),y)
 IPTABLES_CONF_OPT += --enable-ipv6=no
 endif
@@ -16,7 +16,6 @@ IPTABLES_INSTALL_TARGET = YES
 
 IPTABLES_AUTORECONF = YES
 IPTABLES_DEPENDENCIES =
-IPTABLES_CONFIGURE_OPT = --with-kernel=$(LINUX_HEADERS_DIR)
 
 $(eval $(call AUTOTARGETS,package,iptables))