s390/irq: Only define synchronize_irq() on SMP

In uniprocessor configurations, synchronize_irq() is defined in
<linux/hardirq.h> as a macro, and this function definition fails to
compile.
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: stable@vger.kernel.org # 3.9
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/irq.c

@@ -312,6 +312,7 @@ void measurement_alert_subclass_unregister(void)
 }
 EXPORT_SYMBOL(measurement_alert_subclass_unregister);
 
+#ifdef CONFIG_SMP
 void synchronize_irq(unsigned int irq)
 {
 	/*
@@ -320,6 +321,7 @@ void synchronize_irq(unsigned int irq)
 	 */
 }
 EXPORT_SYMBOL_GPL(synchronize_irq);
+#endif
 
 #ifndef CONFIG_PCI