Fix date format specification in wxDatePickerCtrlGeneric with wxUSE_INTL=0

Conversion specification for preferred date representation is "%x", not "x".

src/generic/datectlg.cpp

@@ -252,7 +252,7 @@ private:
 
         return fmt;
 #else // !wxUSE_INTL
-        return wxT("x");
+        return wxS("%x");
 #endif // wxUSE_INTL/!wxUSE_INTL
     }