[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>

arch/s390/kernel/ipl.c

@@ -23,6 +23,7 @@
 #include <asm/ebcdic.h>
 #include <asm/reset.h>
 #include <asm/sclp.h>
+#include <asm/sigp.h>
 #include <asm/checksum.h>
 
 #define IPL_PARM_BLOCK_VERSION 0