Avoid bug in VS 2015 code generation

When using Whole Program Optimization, pItem->OnDrawItem() incorrectly calls the
base class wxOwnerDrawn::OnDrawItem() instead of the appropriate override, for
example wxCheckListBoxItem::OnDrawItem(). The problem can be avoided by not
using an unnecessary cast to wxListBoxItem*.

src/msw/listbox.cpp

@@ -777,7 +777,7 @@ bool wxListBox::MSWOnDraw(WXDRAWITEMSTRUCT *item)
     if ( pStruct->itemID == (UINT)-1 )
         return false;
 
-    wxListBoxItem *pItem = (wxListBoxItem *)m_aItems[pStruct->itemID];
+    wxOwnerDrawn *pItem = m_aItems[pStruct->itemID];
 
     wxDCTemp dc((WXHDC)pStruct->hDC);