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 <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/smp.c

@@ -226,6 +226,8 @@ out:
 	return -ENOMEM;
 }
 
+#ifdef CONFIG_HOTPLUG_CPU
+
 static void pcpu_free_lowcore(struct pcpu *pcpu)
 {
 	pcpu_sigp_retry(pcpu, sigp_set_prefix, 0);
@@ -247,6 +249,8 @@ static void pcpu_free_lowcore(struct pcpu *pcpu)
 	}
 }
 
+#endif /* CONFIG_HOTPLUG_CPU */
+
 static void pcpu_prepare_secondary(struct pcpu *pcpu, int cpu)
 {
 	struct _lowcore *lc = pcpu->lowcore;