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>

arch/powerpc/kernel/sysfs.c

@@ -717,9 +717,11 @@ static void unregister_cpu_online(unsigned int cpu)
 
 	BUG_ON(!c->hotpluggable);
 
+#ifdef CONFIG_PPC64
 	if (!firmware_has_feature(FW_FEATURE_ISERIES) &&
 			cpu_has_feature(CPU_FTR_SMT))
 		sysdev_remove_file(s, &attr_smt_snooze_delay);
+#endif
 
 	/* PMC stuff */
 	switch (cur_cpu_spec->pmc_type) {