tracing: Allow disabling compilation of specific trace systems

Allow a trace events header file to disable compilation of its
trace events by defining the preprocessor macro NOTRACE.

This could be done, for example, according to a Kconfig option.

Link: http://lkml.kernel.org/r/1438432079-11704-3-git-send-email-tal.shorer@gmail.com



Signed-off-by: Tal Shorer <tal.shorer@gmail.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>