Fix php strict error for a function which is always called statically.

Ran into an issue during local development:

    Strict standards: Non-static method EventOrganiser_Shortcodes::eo_clean_input() should not be called statically in /Users/jaspervanwanrooy/development/vht-opleidingen/wp-content/plugins/event-organiser/classes/class-eventorganiser-shortcodes.php on line 255

Given the fact that this function is only called statically, let's also declare it statically.