[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>

arch/s390/kernel/entry.S

@@ -1079,8 +1079,10 @@ cleanup_io_leave_insn:
 .Lexecve_tail:	.long	execve_tail
 .Ljump_table:	.long	pgm_check_table
 .Lschedule:	.long	schedule
+#ifdef CONFIG_PREEMPT
 .Lpreempt_schedule_irq:
 		.long	preempt_schedule_irq
+#endif
 .Ltrace:	.long	syscall_trace
 .Lschedtail:	.long	schedule_tail
 .Lsysc_table:	.long	sys_call_table