add const to jack_deliver_event() for no particular reason

include/engine.h

@@ -254,7 +254,7 @@ int     jack_stop_freewheeling (jack_engine_t* engine, int engine_exiting);
 jack_client_internal_t *
 jack_client_by_name (jack_engine_t *engine, const char *name);
 
-int  jack_deliver_event (jack_engine_t *, jack_client_internal_t *, jack_event_t *, ...);
+int  jack_deliver_event (jack_engine_t *, jack_client_internal_t *, const jack_event_t *, ...);
 
 void
 jack_engine_signal_problems (jack_engine_t* engine);