tracing: show event name in trace for TRACE_EVENT created events

Unlike TRACE_FORMAT() macros, the TRACE_EVENT() macros do not show
the event name in the trace file. Knowing the event type in the trace
output is very useful.

Instead of:

   task swapper:0 [140] ==> ntpd:3308 [120]

We now have:

   sched_switch: task swapper:0 [140] ==> ntpd:3308 [120]

Signed-off-by: Steven Rostedt <srostedt@redhat.com>