sched: add declaration of sched_tail to sched.h

Avoids sparse warnings:
kernel/sched.c:2170:17: warning: symbol 'schedule_tail' was not declared. Should it be static?

Avoids the need for an external declaration in arch/um/process.c

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/include/linux/sched.h b/include/linux/sched.h
index e217d18..9c17e82 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -242,6 +242,7 @@
 
 extern void sched_init(void);
 extern void sched_init_smp(void);
+extern asmlinkage void schedule_tail(struct task_struct *prev);
 extern void init_idle(struct task_struct *idle, int cpu);
 extern void init_idle_bootup_task(struct task_struct *idle);