xen/gntdev: Use select for DMA_SHARED_BUFFER

DMA_SHARED_BUFFER can not be enabled by the user (it represents a library
set in the kernel). The kconfig convention is to use select for such
symbols so they are turned on implicitly when the user enables a kconfig
that needs them.

Otherwise the XEN_GNTDEV_DMABUF kconfig is overly difficult to enable.

Fixes: 932d6562

 ("xen/gntdev: Add initial support for dma-buf UAPI")
Cc: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: xen-devel@lists.xenproject.org
Cc: Juergen Gross <jgross@suse.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Juergen Gross <jgross@suse.com>