Fix drop down combobox flags in the widgets sample

Combine, not replace, combobox option flags when creating a drop down one,
otherwise all the others flags were simply ignored.

Closes https://github.com/wxWidgets/wxWidgets/pull/136

samples/widgets/combobox.cpp

@@ -431,7 +431,7 @@ void ComboboxWidgetsPage::CreateCombo()
             break;
 
         case ComboKind_DropDown:
-            flags = wxCB_DROPDOWN;
+            flags |= wxCB_DROPDOWN;
             break;
     }