vringh: pull in spinlock header

we use a spinlock now pull in the correct header to
make vring.h self sufficient.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

include/linux/vringh.h

@@ -14,6 +14,7 @@
 #include <linux/virtio_byteorder.h>
 #include <linux/uio.h>
 #include <linux/slab.h>
+#include <linux/spinlock.h>
 #if IS_REACHABLE(CONFIG_VHOST_IOTLB)
 #include <linux/dma-direction.h>
 #include <linux/vhost_iotlb.h>