Fix gcc 4.1.2 compile warnings

src/V3Gate.cpp

@@ -64,6 +64,7 @@ class GateGraphBaseVisitor {
 public:
     virtual AstNUser* visit(GateLogicVertex* vertexp, AstNUser* vup=NULL) =0;
     virtual AstNUser* visit(GateVarVertex* vertexp, AstNUser* vup=NULL) =0;
+    virtual ~GateGraphBaseVisitor() {}
 };
 
 //######################################################################

src/V3Hashed.h

@@ -48,6 +48,8 @@ public:
 struct V3HashedUserCheck {
     // Functor for V3Hashed::findDuplicate
     virtual bool check(AstNode*,AstNode*) =0;
+    V3HashedUserCheck() {}
+    virtual ~V3HashedUserCheck() {}
 };
 
 class V3Hashed : public VHashedBase {