fix use of int or unsigned long instead of int for monitor id

src/zm_eventstream.h

@@ -52,7 +52,7 @@ class EventStream : public StreamBase {
 
     struct EventData {
       uint64_t  event_id;
-      unsigned long   monitor_id;
+      unsigned int    monitor_id;
       unsigned long   storage_id;
       unsigned long   frame_count;
       time_t          start_time;