fix recently broken gtk a11y

since

commit aa6c54f337
Date:   Mon Jan 18 09:01:15 2016 +0200

    loplugin:unusedfields

structure is this way for gobject style inheritence

Change-Id: Ia069333aa5cfd913a2e8f85c967dabdcf4de6615

vcl/unx/gtk/a11y/atkwrapper.hxx

@@ -45,6 +45,8 @@ namespace com { namespace sun { namespace star { namespace accessibility {
 
 struct _AtkObjectWrapper
 {
+    AtkObject aParent;
+
     css::uno::Reference<css::accessibility::XAccessible> mpAccessible;
     css::uno::Reference<css::accessibility::XAccessibleContext> mpContext;
     css::uno::Reference<css::accessibility::XAccessibleAction> mpAction;