MDL-50254 Mustache: Missing alias for namespaced class in mustache

Thanks to Francesco Bigiarini for the report and suggested fix.

lib/classes/output/mustache_template_finder.php

@@ -28,6 +28,7 @@ namespace core\output;
 use coding_exception;
 use moodle_exception;
 use core_component;
+use theme_config;
 
 /**
  * Get information about valid locations for mustache templates.