Minor bug fix for flow event timestamp

The google trace event format leaves a little bit to be desired,
in that the start of a flow event has to be between the start end end
of the parent task, not equal to the start. So, we take the start
timestamp of the parent and add 0.250 microseconds which is the
smallest resolution we can use to increase the timestamp to make
sure it is during the parent but not after the parent stopped.

src/apex/trace_event_listener.cpp

@@ -218,7 +218,7 @@ inline void trace_event_listener::_common_stop(std::shared_ptr<profiler> &p) {
             ) {
             //std::cout << "FLOWING!" << std::endl;
             uint64_t flow_id = reversed_node_id + get_flow_id();
-            write_flow_event(ss, p->tt_ptr->parent->get_flow_us(), 's', "ControlFlow", flow_id,
+            write_flow_event(ss, p->tt_ptr->parent->get_flow_us()+0.250, 's', "ControlFlow", flow_id,
                 saved_node_id, p->tt_ptr->parent->thread_id, p->tt_ptr->parent->task_id->get_name());
             write_flow_event(ss, p->get_start_us(), 'f', "ControlFlow", flow_id,
                 saved_node_id, _tid, p->tt_ptr->parent->task_id->get_name());