remoteproc: select VIRTIO to avoid build breakage

drivers/built-in.o: In function `rproc_virtio_finalize_features':
remoteproc_virtio.c:(.text+0x2f9a02): undefined reference to `vring_transport_features'
drivers/built-in.o: In function `rproc_virtio_del_vqs':
remoteproc_virtio.c:(.text+0x2f9a74): undefined reference to `vring_del_virtqueue'
drivers/built-in.o: In function `rproc_virtio_find_vqs':
remoteproc_virtio.c:(.text+0x2f9c44): undefined reference to `vring_new_virtqueue'
drivers/built-in.o: In function `rproc_add_virtio_dev':
(.text+0x2f9e2c): undefined reference to `register_virtio_device'
drivers/built-in.o: In function `rproc_vq_interrupt':
(.text+0x2f9db7): undefined reference to `vring_interrupt'
drivers/built-in.o: In function `rproc_remove_virtio_dev':
(.text+0x2f9e9f): undefined reference to `unregister_virtio_device'

Cc: stable@vger.kernel.org
Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>