[S390] ipl: fix compile breakage

Fix this for !CONFIG_SMP:

arch/s390/kernel/ipl.c: In function 'stop_run':
arch/s390/kernel/ipl.c:1461: error: implicit declaration of function 'signal_processor'

Reported-by: Sachin Sant <sachinp@in.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>