sched, ftrace: trace sched.c

Impact: allow function tracing within sched.c

Its useful to see what happens in sched.c.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>