DER encoding: Add stub for SimpleVisitor implementation.
diff --git a/ndn-cpp/encoding/der/visitor/simple-visitor.hpp b/ndn-cpp/encoding/der/visitor/simple-visitor.hpp
index 4ec3a8c..d54c499 100644
--- a/ndn-cpp/encoding/der/visitor/simple-visitor.hpp
+++ b/ndn-cpp/encoding/der/visitor/simple-visitor.hpp
@@ -15,7 +15,7 @@
 
 namespace der {
 
-class NoArgumentsVisitor : public NoArgumentsVisitor
+class SimpleVisitor : public NoArgumentsVisitor
 {
 public:
   virtual Any visit(DerBool&           ) = 0;