Fix userland build broken by r360292.

sys/net/route.h

@@ -331,6 +331,7 @@ struct rt_msghdr {
 #define RTAX_BRD	7	/* for NEWADDR, broadcast or p-p dest addr */
 #define RTAX_MAX	8	/* size of array to allocate */
 
+struct rtentry;
 struct nhop_object;
 typedef int rt_filter_f_t(const struct rtentry *, const struct nhop_object *,
     void *);