s390/ap_bus: add missing ifdef

Get rid of this compile warning for CONFIG_32BIT:

drivers/s390/crypto/ap_bus.c:168:12: warning: ‘ap_configuration_available’
  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>

drivers/s390/crypto/ap_bus.c

@@ -165,10 +165,12 @@ static int ap_interrupts_available(void)
  *
  * Returns 1 if AP configuration information is available.
  */
+#ifdef CONFIG_64BIT
 static int ap_configuration_available(void)
 {
 	return test_facility(2) && test_facility(12);
 }
+#endif
 
 /**
  * ap_test_queue(): Test adjunct processor queue.