C++11: Use override where possible. No functional change.

src/V3Cast.cpp

@@ -193,7 +193,7 @@ private:
 public:
     // CONSTRUCTORS
     explicit CastVisitor(AstNetlist* nodep) { iterate(nodep); }
-    virtual ~CastVisitor() {}
+    virtual ~CastVisitor() override {}
 };
 
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