sched: Export account_system_vtime()

KVM uses it for example:

 ERROR: "account_system_vtime" [arch/x86/kvm/kvm.ko] undefined!

Cc: Venkatesh Pallipadi <venki@google.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1286237003-12406-3-git-send-email-venki@google.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

kernel/sched.c

@@ -1991,6 +1991,7 @@ void account_system_vtime(struct task_struct *curr)
 
 	local_irq_restore(flags);
 }
+EXPORT_SYMBOL_GPL(account_system_vtime);
 
 static void sched_irq_time_avg_update(struct rq *rq, u64 curr_irq_time)
 {