virtio_ring: pull in spinlock header

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

drivers/virtio/virtio_ring.c

@@ -11,6 +11,7 @@
 #include <linux/module.h>
 #include <linux/hrtimer.h>
 #include <linux/dma-mapping.h>
+#include <linux/spinlock.h>
 #include <xen/xen.h>
 
 #ifdef DEBUG