kernel/sched: don't expose local functions

The get_rr_interval_* functions are all class methods of
struct sched_class. They are not exported so make them
static.
Signed-off-by: NH Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Peter Zijlstra <peterz@infradead.org>
LKML-Reference: <201001132021.53253.hartleys@visionengravers.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>