Handle the removal of tracing_enabled file

The file tracing_enabled, once deprecated, has been completely removed
in kernel 3.7.rc3.
Handle this by adding a control over the kernel release number.

Signed-off-by: Serena Ziviani <senseriumi@gmail.com>
Reviewed-by: Arianna Avanzini <avanzini.arianna@gmail.com>
wrapper.c: added comment on the overall program

Signed-off-by: Serena Ziviani <senseriumi@gmail.com>
Reviewed-by: Arianna Avanzini <avanzini.arianna@gmail.com>