Use bool for evregpy_no_listeners_p

I noticed that evregpy_no_listeners_p should return a bool.  This
patch makes this change.  I'm checking it in.

gdb/python/py-events.h

@@ -52,6 +52,6 @@ struct events_object
 extern events_object gdb_py_events;
 
 extern eventregistry_object *create_eventregistry_object (void);
-extern int evregpy_no_listeners_p (eventregistry_object *registry);
+extern bool evregpy_no_listeners_p (eventregistry_object *registry);
 
 #endif /* PYTHON_PY_EVENTS_H */

gdb/python/py-evtregistry.c

@@ -115,7 +115,7 @@ gdbpy_initialize_eventregistry (void)
 /* Return the number of listeners currently connected to this
    registry.  */
 
-int
+bool
 evregpy_no_listeners_p (eventregistry_object *registry)
 {
   return PyList_Size (registry->callbacks) == 0;