MDL-42706 mod_book: Undefined CFG in pluginfile function

mod/book/lib.php

@@ -405,7 +405,7 @@ function book_get_file_info($browser, $areas, $course, $cm, $context, $filearea,
  * @return bool false if file not found, does not return if found - just send the file
  */
 function book_pluginfile($course, $cm, $context, $filearea, $args, $forcedownload, array $options=array()) {
-    global $DB;
+    global $CFG, $DB;
 
     if ($context->contextlevel != CONTEXT_MODULE) {
         return false;