[PATCH] powerpc: fix for compile problem in kdump code when SMP disabled

This patch fixes the compilation error (shown below) when CONFIG_SMP=n.
    arch/powerpc/kernel/crash.c: In function `crash_kexec_prepare_cpus':
    arch/powerpc/kernel/crash.c:236: error: implicit declaration of
    function `smp_release_cpus'

Signed-off-by: Haren Myneni <haren@us.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>