ftrace: user proper API for setting RT prios in selftest

The wakeup selftest used an internal API for setting the test task priority.
This patch fixes it to use the proper API for performing such a task.

Thanks goes to Randy Dunlap for pointing out this build failure.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>