Replace comma with semicolon in trace/simple.c

In the master branch in trace/simple.c in writeout_thread (https://github.com/qemu/qemu/blob/master/trace/simple.c#L174) we currently have:
  dropped.rec.length = sizeof(TraceRecord) + sizeof(uint64_t),
  dropped.rec.pid = trace_pid;

It seems to me like a typo that the first line ends with a comma.
Currently this causes no harm, but I think this should be fixed.