CLASSIFY is now built unconditionally, thus we need the kernel header
diff --git a/include/linux/netfilter_ipv4/ipt_CLASSIFY.h b/include/linux/netfilter_ipv4/ipt_CLASSIFY.h
new file mode 100644
index 0000000..7596e3d
--- /dev/null
+++ b/include/linux/netfilter_ipv4/ipt_CLASSIFY.h
@@ -0,0 +1,8 @@
+#ifndef _IPT_CLASSIFY_H
+#define _IPT_CLASSIFY_H
+
+struct ipt_classify_target_info {
+	u_int32_t priority;
+};
+
+#endif /*_IPT_CLASSIFY_H */