vfio-mdev: allow building the samples into the kernel

There is nothing in the vfio-mdev sample drivers that requires building
them as modules, so remove that restriction.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Tony Krowiak <akrowiak@linux.ibm.com>
Link: https://lore.kernel.org/r/20230110091009.474427-2-hch@lst.de


Signed-off-by: Alex Williamson <alex.williamson@redhat.com>