ftrace: Allow other users of function tracing to use the output listing

The function tracer is set up to allow any other subsystem (like perf)
to use it. Ftrace already has a way to list what functions are enabled
by the global_ops. It would be very helpful to let other users of
the function tracer to be able to use the same code.

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