Allow using column sort indicators in virtual wxListCtrl too

There doesn't seem to be any reason to not show sort indicators in the
columns of virtual list controls, so simply remove the test for
IsVirtual() in DrawSortArrow().

This allows d8ec0aa001 (Support sort indicators in wxListCtrl header,
2021-11-28) to work for wxListCtrl with wxLC_VIRTUAL too.

src/generic/listctrl.cpp

@@ -1104,7 +1104,7 @@ void wxListHeaderWindow::OnPaint( wxPaintEvent &WXUNUSED(event) )
 #endif
 
         wxHeaderSortIconType sortArrow = wxHDR_SORT_ICON_NONE;
-        if ( !m_owner->IsVirtual() && m_enableSortCol && i == m_sortCol )
+        if ( m_enableSortCol && i == m_sortCol )
         {
             if ( m_sortAsc )
                 sortArrow = wxHDR_SORT_ICON_UP;