The VFIO_AP uses ap_driver_register() (and deregister) functions
implemented in ap_bus.c (compiled into ap.o).  However the ap.o will be
built only if CONFIG_ZCRYPT is selected.

This was not visible before commit e93a1695 ("iommu: Enable compile
testing for some of drivers") because the CONFIG_VFIO_AP depends on
CONFIG_S390_AP_IOMMU which depends on the missing CONFIG_ZCRYPT.  After
adding COMPILE_TEST, it is possible to select a configuration with
VFIO_AP and S390_AP_IOMMU but without the ZCRYPT.

Add proper dependency to the VFIO_AP to fix build errors:

ERROR: modpost: "ap_driver_register" [drivers/s390/crypto/vfio_ap.ko] undefined!
ERROR: modpost: "ap_driver_unregister" [drivers/s390/crypto/vfio_ap.ko] undefined!
Reported-by: kernel test robot <lkp@intel.com>
Fixes: e93a1695 ("iommu: Enable compile testing for some of drivers")
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>