Use wxASCII_STR() on string literals

Fix the build with wxNO_IMPLICIT_WXSTRING_ENCODING.

include/wx/msw/richmsgdlg.h

@@ -15,7 +15,7 @@ class WXDLLIMPEXP_CORE wxRichMessageDialog : public wxGenericRichMessageDialog
 public:
     wxRichMessageDialog(wxWindow *parent,
                         const wxString& message,
-                        const wxString& caption = wxMessageBoxCaptionStr,
+                        const wxString& caption = wxASCII_STR(wxMessageBoxCaptionStr),
                         long style = wxOK | wxCENTRE)
         : wxGenericRichMessageDialog(parent, message, caption, style)
         { }