routing: made map more container-agnostic

Change-Id: Ib18a41bac7627a9eb0bbafa6ccad0335d420dd68
refs: #4239
diff --git a/src/adjacency-list.hpp b/src/adjacency-list.hpp
index dcb1539..c21e70b 100644
--- a/src/adjacency-list.hpp
+++ b/src/adjacency-list.hpp
@@ -70,6 +70,9 @@
   std::list<Adjacent>&
   getAdjList();
 
+  const std::list<Adjacent>&
+  getAdjList() const;
+
   bool
   isNeighbor(const ndn::Name& adjName);