Class 'CI_Controller' not found

Never modify any files in the system folder or subfolders.
Only create or modify files in the application folder and it's subfolders.

Thanks for the answer. But I need to modify the method in core/Common.php
I get in this method via a global variable $_SERVER the remote ip-address and I need to get data from database or from session and to record in log message.
Therefore, for me, the best way out would be to connect the session library to this method and get the data. But I think core/Controller declares later than Common.php and therefore I getting this error.

I don't see that any of your requirements cannot be accomplished in the controller's constructor. You're going to have to provide more code and strong arguments for hacking the core.