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>