sched: fix !CONFIG_SCHEDSTATS build failure

Stephen Rothwell reported this linux-next build failure with !CONFIG_SCHEDSTATS:

| In file included from kernel/sched.c:1703:
| kernel/sched_fair.c: In function 'adaptive_gran':
| kernel/sched_fair.c:1324: error: 'struct sched_entity' has no member named 'avg_wakeup'

The start_runtime and avg_wakeup metrics are now not just for statistics,
but also for scheduling - so they always need to be available. (Also
move out the nr_migrations fields - for future perfcounters usage.)

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Ingo Molnar <mingo@elte.hu>