Fix sim build failure

Fix sim build failure

On x86-64 Fedora 32, the sim was failing to build.
sim_events_schedule was passing a 'dummy' argument to
sim_events_schedule_vtracef, which caused an error because the format
parameter was NULL.  However, removing this dummy argument caused an
error because too few arguments were being passed -- catch 22.

This patch fixes the build problem by using sim_events_schedule_tracef
instead.

sim/common/ChangeLog
2021-04-22  Tom Tromey  <tom@tromey.com>

	* sim-events.c (sim_events_schedule): Use
	sim_events_schedule_tracef.