QmlJS: Make destructor of base class virtual

Change-Id: I11233c4fde6e1db09b604965f07ae32741c1bddf
Reviewed-by: Marco Benelli <marco.benelli@qt.io>

src/libs/qmljs/qmljsvalueowner.h

@@ -63,7 +63,7 @@ class QMLJS_EXPORT ValueOwner
 public:
     static SharedValueOwner *sharedValueOwner(QString kind = QString());
     ValueOwner(const SharedValueOwner *shared = 0);
-    ~ValueOwner();
+    virtual ~ValueOwner();
 
     const NullValue *nullValue() const;
     const UndefinedValue *undefinedValue() const;