tracing: Add __used annotation to event variable

The TRACE_EVENT() macros automate creation of trace events. To automate
initialization, the set up variables are loaded in a special section
that is read on boot up. GCC is not aware that these static variables
are used and will complain about them if we do not inform GCC that
they are indeed used.

One of the declarations of the event element was missing a __used
annotation. This patch adds it.

Reported-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>