MDL-46455 broke backup and restore of subplugins.

If your subplugin backup file does a require_once, e.g. https://github.com/moodleou/moodle-quizaccess_honestycheck/blob/master/backup/moodle2/backup_quizaccess_honestycheck_subplugin.class.php does

require_once($CFG->dirroot . '/mod/quiz/backup/moodle2/backup_mod_quiz_access_subplugin.class.php');

then since MDL-46455 you get a fatal error, since the new add_subplugin_structure function does not declare global $CFG. (The previous implementation, before that change, did.)