s390/smp: get rid of compile warning

Add missing #ifdep CONFIG_HOTPLUG_CPU to get rid of this one:
arch/s390/kernel/smp.c:229:13: warning: 'pcpu_free_lowcore'
defined but not used [-Wunused-function]

Signed-off-by: Heiko Carstens <[e-mail]>
Signed-off-by: Martin Schwidefsky <[e-mail]>

arch/s390/kernel/smp.c

···
 	return -ENOMEM;
 }
 
+#ifdef CONFIG_HOTPLUG_CPU
+
 static void pcpu_free_lowcore(struct pcpu *pcpu)
 {
 	pcpu_sigp_retry(pcpu, sigp_set_prefix, 0);
···
 		free_pages((unsigned long) pcpu->lowcore, LC_ORDER);
 	}
 }
+
+#endif /* CONFIG_HOTPLUG_CPU */
 
 static void pcpu_prepare_secondary(struct pcpu *pcpu, int cpu)
 {