vfio-mdev: add back CONFIG_VFIO dependency

CONFIG_VFIO_MDEV cannot be selected when VFIO itself is
disabled, otherwise we get a link failure:

WARNING: unmet direct dependencies detected for VFIO_MDEV
  Depends on [n]: VFIO [=n]
  Selected by [y]:
  - SAMPLE_VFIO_MDEV_MTTY [=y] && SAMPLES [=y]
  - SAMPLE_VFIO_MDEV_MDPY [=y] && SAMPLES [=y]
  - SAMPLE_VFIO_MDEV_MBOCHS [=y] && SAMPLES [=y]
/home/arnd/cross/arm64/gcc-13.0.1-nolibc/x86_64-linux/bin/x86_64-linux-ld: samples/vfio-mdev/mdpy.o: in function `mdpy_remove':
mdpy.c:(.text+0x1e1): undefined reference to `vfio_unregister_group_dev'
/home/arnd/cross/arm64/gcc-13.0.1-nolibc/x86_64-linux/bin/x86_64-linux-ld: samples/vfio-mdev/mdpy.o: in function `mdpy_probe':
mdpy.c:(.text+0x149e): undefined reference to `_vfio_alloc_device'

Fixes: 8bf8c5ee

 ("vfio-mdev: turn VFIO_MDEV into a selectable symbol")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20230126211211.1762319-1-arnd@kernel.org
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>