src: improve constness

Change-Id: I8b3084e311642578628ea7c1017dc2a58979ab83
refs: #4472
diff --git a/src/nlsr.hpp b/src/nlsr.hpp
index 0a29184..d3b8737 100644
--- a/src/nlsr.hpp
+++ b/src/nlsr.hpp
@@ -123,18 +123,36 @@
     return m_confParam;
   }
 
+  const ConfParameter&
+  getConfParameter() const
+  {
+    return m_confParam;
+  }
+
   AdjacencyList&
   getAdjacencyList()
   {
     return m_adjacencyList;
   }
 
+  const AdjacencyList&
+  getAdjacencyList() const
+  {
+    return m_adjacencyList;
+  }
+
   NamePrefixList&
   getNamePrefixList()
   {
     return m_namePrefixList;
   }
 
+  const NamePrefixList&
+  getNamePrefixList() const
+  {
+    return m_namePrefixList;
+  }
+
   ndn::Face&
   getNlsrFace()
   {