Use using directives to silence hiding warning

src/parser/check_refs.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015, Intel Corporation
+ * Copyright (c) 2015-2016, Intel Corporation
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are met:
@@ -73,6 +73,8 @@ public:
         throw ParseError(str.str());
     }
 
+    using DefaultConstComponentVisitor::pre;
+
     void pre(const ComponentBackReference &c) override {
         if (c.ref_id) {
             if (c.ref_id >= num_ids) {