powerpc: Fix build for 32-bit SMP configs

attr_smt_snooze_delay is only defined for CONFIG_PPC64, so protect the
attribute removal with the same condition.  This fixes this build error
on 32-bit SMP configurations:

/data/home/miltonm/next.git/arch/powerpc/kernel/sysfs.c: In function ‘unregister_cpu_online’:
/data/home/miltonm/next.git/arch/powerpc/kernel/sysfs.c:722: error: ‘attr_smt_snooze_delay’ undeclared (first use in this function)
/data/home/miltonm/next.git/arch/powerpc/kernel/sysfs.c:722: error: (Each undeclared identifier is reported only once
/data/home/miltonm/next.git/arch/powerpc/kernel/sysfs.c:722: error: for each function it appears in.)

Signed-off-by: Paul Mackerras <paulus@samba.org>