tracing: Add TRACE_EVENT_FN example

If a function should be called before a tracepoint is enabled
and/or after it is disabled, the TRACE_EVENT_FN() serves this
purpose. But it is not well documented. Having it as a sample would
help developers to know how to use it.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

diff --cc samples/trace_events/trace-events-sample.c
Simple merge

diff --cc samples/trace_events/trace-events-sample.h
Simple merge