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: NHarvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>