virtio: add missing MODULE_DESCRIPTION() macro

make allmodconfig && make W=1 C=1 reports:
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/virtio/virtio_dma_buf.o

Add the missing invocation of the MODULE_DESCRIPTION() macro.

Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Message-Id: <20240602-md-virtio_dma_buf-v1-1-ce602d47e257@quicinc.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>