sched: Add #ifdef around irq time accounting functions

Get rid of this:

 kernel/sched.c:3731:13: warning: 'irqtime_account_idle_ticks' defined but not used
 kernel/sched.c:3732:13: warning: 'irqtime_account_process_tick' defined but not used

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Venkatesh Pallipadi <venki@google.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20110225133228.GD7469@osiris.boeblingen.de.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>