Address review comments

Author: fabiomestre

sycl/source/detail/graph_impl.cpp

@@ -1026,7 +1026,8 @@ exec_graph_impl::enqueue(const std::shared_ptr<sycl::detail::queue_impl> &Queue,
            It != MExecutionEvents.end();) {
         std::shared_ptr<event_impl> &Event = *It;
         if (!Event->isCompleted()) {
-          auto &AttachedEventsList = Event->getPostCompleteEvents();
+          const std::vector<EventImplPtr> &AttachedEventsList =
+              Event->getPostCompleteEvents();
           CGData.MEvents.reserve(CGData.MEvents.size() +
                                  AttachedEventsList.size() + 1);
           CGData.MEvents.push_back(Event);