Fix the discrepancy between cfg_pluginlist_foreach declaration and definition

There was a missing indirection for the pluginlist_cb_t *callback in the
declaration of the cfg_pluginlist_foreach function.  Reported by MSVC as:

  lib\isccfg\parser.c(4057): warning C4028: formal parameter 4 different from declaration

(cherry picked from commit 4ffe725585)

lib/isccfg/include/isccfg/cfg.h

@@ -597,7 +597,7 @@ typedef isc_result_t(pluginlist_cb_t)(const cfg_obj_t *config,
 
 isc_result_t
 cfg_pluginlist_foreach(const cfg_obj_t *config, const cfg_obj_t *list,
-		       isc_log_t *lctx, pluginlist_cb_t callback,
+		       isc_log_t *lctx, pluginlist_cb_t *callback,
 		       void *callback_data);
 /*%<
  * For every "plugin" stanza present in 'list' (which in turn is a part of