How to stop Watchdog from logging "DEBUG" errors?
The Drupal watchdog includes the errors raised by PHP, such as the one you mention.

It might be possible to use hook_watchdog to filter out certain errors. However, it seems easier to change the PHP error level on your settings.php file, so that PHP does not complain about your ctools module.


The list of possible PHP error levels is here: http://php.net/manual/en/errorfunc.constants.php
Some guidance about modifying the error levels on your settings.phpfile: https://www.drupal.org/docs/7/creating-custom-modules/show-all-errors-while-developing