Make it possible to use controls inside wxComboPopup again

This should have been done together with the changes of 41410610ef
(Don't force wxPU_CONTAINS_CONTROLS on wxPopupTransientWindow,
2020-07-10) as without this style controls inside wxComboPopup couldn't
accept focus any more, which broke the previously working code.

src/common/combocmn.cpp

@@ -454,7 +454,8 @@ public:
     wxComboPopupWindow( wxComboCtrlBase *parent,
                         int style )
     #if USES_WXPOPUPWINDOW || USES_WXPOPUPTRANSIENTWINDOW
-                       : wxComboPopupWindowBase(parent,style)
+                       : wxComboPopupWindowBase(parent,
+                                                style | wxPU_CONTAINS_CONTROLS)
     #else
                        : wxComboPopupWindowBase(parent,
                                                 wxID_ANY,