output-json-flow: add has_alerts field

Add has_alerts field to flow eve-log to indicate if a flow has
any alerts or not.

src/output-json-flow.c

@@ -261,6 +261,8 @@ static void JsonFlowLogJSON(JsonFlowLogThread *aft, json_t *js, Flow *f)
     json_object_set_new(hjs, "reason",
             json_string(reason));
 
+    json_object_set_new(hjs, "alerted", json_boolean(FlowHasAlerts(f)));
+
     json_object_set_new(js, "flow", hjs);