ctools_export_ui_list_form argument should be passed by reference.

Form builder functions should be passed $form_state by reference. But one function in ctools_export_ui.class.php doesn't.

This bug is subtle and shows up in Stylizer where the listing of styles complains of a missing plugin and an unknown style base fore very style. The reason is that ctools_export_ui_list_form() is not getting the $form_state by reference. In stylizer's list_form method, the style bases are loaded and stashed in the form state. Since that is not passed by reference, they're not in the form state when the table rows are being set up.