rib: do the longest-prefix match in self-learning route renew helper

Change-Id: I7f6f349fda6925dc3847716fed123d745f11b02a
refs: #4683
diff --git a/rib/rib.hpp b/rib/rib.hpp
index 5beea2d..a9278de 100644
--- a/rib/rib.hpp
+++ b/rib/rib.hpp
@@ -77,6 +77,9 @@
   Route*
   find(const Name& prefix, const Route& route) const;
 
+  Route*
+  findLongestPrefix(const Name& prefix, const Route& route) const;
+
   const_iterator
   begin() const;