Make wxTextCtrlOleCallback destructor virtual to avoid g++ warnings.

This class doesn't really need a virtual dtor as it's never used
polymorphically, but add it to avoid g++ warnings about it.

(this is a backport of 8ad1e2698f from master)

src/msw/textctrl.cpp

@@ -169,7 +169,7 @@ class wxTextCtrlOleCallback : public IRichEditOleCallback
 {
 public:
     wxTextCtrlOleCallback(wxTextCtrl *text) : m_textCtrl(text), m_menu(NULL) {}
-    ~wxTextCtrlOleCallback() { DeleteContextMenuObject(); }
+    virtual ~wxTextCtrlOleCallback() { DeleteContextMenuObject(); }
 
     STDMETHODIMP ContextSensitiveHelp(BOOL WXUNUSED(enterMode)) { return E_NOTIMPL; }
     STDMETHODIMP DeleteObject(LPOLEOBJECT WXUNUSED(oleobj)) { return E_NOTIMPL; }