ipsec-tools: fix typo in variable name

As pointed out during the check-package discussion, there is a typo in a
variable name:

http://lists.busybox.net/pipermail/buildroot/2016-December/180765.html

Reported-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/ipsec-tools/ipsec-tools.mk

@@ -57,7 +57,7 @@ endif
 ifneq ($(BR2_PACKAGE_IPSEC_TOOLS_READLINE),y)
 IPSEC_TOOLS_CONF_OPTS += --without-readline
 else
-IPSEC_DEPENDENCIES += readline
+IPSEC_TOOLS_DEPENDENCIES += readline
 endif
 
 ifeq ($(BR2_PACKAGE_IPSEC_TOOLS_HYBRID),y)