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>

samples/Kconfig

@@ -135,6 +135,7 @@ config SAMPLE_VFIO_MDEV_MDPY_FB
 config SAMPLE_VFIO_MDEV_MBOCHS
 	tristate "Build VFIO mdpy example mediated device sample code -- loadable modules only"
 	depends on VFIO_MDEV_DEVICE && m
+	select DMA_SHARED_BUFFER
 	help
 	  Build a virtual display sample driver for use as a VFIO
 	  mediated device.  It supports the region display interface