samples: mbochs: add DMA_SHARED_BUFFER dependency

The new bochs vbe sample fails to link when DMA_SHARED_BUFFER is
disabled:

ERROR: "dma_buf_export" [samples/vfio-mdev/mbochs.ko] undefined!
ERROR: "dma_buf_fd" [samples/vfio-mdev/mbochs.ko] undefined!

This uses a 'select' statement to enable that framework, like all
other users do.

Fixes: a5e6e650 ("sample: vfio bochs vbe display (host device for bochs-drm)")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>