treeview: fix build without accessibility

Amend 6a4afebc5ce8db69a6c9fb398cada31e6bad5e3c and add ifdefs around
use of Accessibility.

Change-Id: Ib40132dabeec9c3cff2f71dd5182eb1715b9c76a
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
(cherry picked from commit 84e6d89c1b07830da82da1073930c094f6b99a81)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/widgets/itemviews/qtreeview.cpp

@@ -3325,6 +3325,7 @@ void QTreeViewPrivate::layout(int i, bool recursiveExpanding, bool afterIsUninit
         return;
     }
 
+#if QT_CONFIG(accessibility)
     // QAccessibleTree's rowCount implementation uses viewItems.size(), so
     // we need to invalidate any cached accessibility data structures if
     // that value changes during the run of this function.
@@ -3334,6 +3335,7 @@ void QTreeViewPrivate::layout(int i, bool recursiveExpanding, bool afterIsUninit
             QAccessible::updateAccessibility(&event);
         }
     });
+#endif
 
     int count = 0;
     if (model->hasChildren(parent)) {