vhost: add virtio crypto header file

vhost: add virtio crypto header file

Since the linux kernel header file virtio_crypto.h has been merged
in 4.9, if we include this header file directly, compilation will be
failed in the old kernels' environment, e.g. the vhost crypto backend
series.
Adding virtio_crypto.h in librte_vhost to make old kernels happy.

Signed-off-by: Jay Zhou <jianjay.zhou@huawei.com>
Signed-off-by: Lei Gong <arei.gonglei@huawei.com>
Acked-by: Fan Zhang <roy.fan.zhang@intel.com>
Acked-by: Maxime Coquelin <maxime.coquelin@redhat.com>