tracing: add back the available_events file

The event directory files type and available_types were no longer
needed with the new TRACE_EVENT_FORMAT macros, they were deleted.
But by accident the available_events file was also removed.
This patch brings it back.

Reported-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Steven Rostedt <srostedt@redhat.com>