don't use MSW-only DEFAULT_ITEM_WIDTH but the width returned by the best class GetBestSize() in wxHeaderCtrl::DoGetBestSize() implementation for empty header

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@57244 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/generic/headerctrlg.cpp

@@ -154,7 +154,7 @@ wxSize wxHeaderCtrl::DoGetBestSize() const
 {
     // the vertical size is rather arbitrary but it looks better if we leave
     // some space around the text
-    return wxSize(IsEmpty() ? DEFAULT_ITEM_WIDTH
+    return wxSize(IsEmpty() ? wxHeaderCtrlBase::DoGetBestSize().x
                             : GetColEnd(GetColumnCount() - 1),
                   (7*GetCharHeight())/4);
 }