Update reader.h

Author: btolfa

Diff for: include/json/reader.h

@@ -270,6 +270,7 @@ class JSON_API CharReader {
 
   class Factory {
   public:
+    virtual ~Factory() {}
     /** \brief Allocate a CharReader via operator new().
      * \throw std::exception if something goes wrong (e.g. invalid settings)
      */