[S390] Fix compile error on 31bit without preemption

Commit b8e7a54c

 introduced a compile
error if CONFIG_PREEMPT is not set:

arch/s390/kernel/built-in.o: In function `cleanup_io_leave_insn':
/space/kvm/arch/s390/kernel/entry.S:(.text+0xbfce): undefined reference to `preempt_schedule_irq'

This patch hides preempt_schedule_irq if CONFIG_PREEMPT is not set.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>