fix another MSVC /Wp64 warning about size_t to int conversion (closes #10140)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_8_BRANCH@56623 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/choicebk.h

@@ -93,7 +93,7 @@ protected:
 
     void UpdateSelectedPage(size_t newsel)
     {
-        m_selection = newsel;
+        m_selection = wx_static_cast(int, newsel);
         GetChoiceCtrl()->Select(newsel);
     }