Add missing __noreturn attribute to cpu_die():

arch/s390/kernel/smp.c:691:6: error: symbol 'cpu_die' redeclared with different type
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>