module: Make make_config() pure virtual

It is important to implement this method in derived classes. When we
provided default implementation returning NULL, introducing new methods
might leave this method not implemented without a compiler error.

Signed-off-by: Simon Rozman <simon@rozman.si>

lib/EAPBase/include/Module.h

@@ -111,7 +111,7 @@ namespace eap
         ///
         /// \returns New method configuration
         ///
-        virtual config_method* make_config();
+        virtual config_method* make_config() = 0;
 
         /// @}