Export class QAccessible::ActivationObserver.

Change a21dfab54e de-inlined its
destructor, so, the class needs to be exported.

Change-Id: I32d21622a944a93718f549060e5e5f7e71fd2646
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>

src/gui/accessible/qaccessible.h

@@ -396,7 +396,7 @@ public:
     static UpdateHandler installUpdateHandler(UpdateHandler);
     static RootObjectHandler installRootObjectHandler(RootObjectHandler);
 
-    class ActivationObserver
+    class Q_GUI_EXPORT ActivationObserver
     {
     public:
         virtual ~ActivationObserver();