tools/virtio: add linux/hrtimer.h stub

Make tool build after virtio changes broke it.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
diff --git a/tools/virtio/linux/hrtimer.h b/tools/virtio/linux/hrtimer.h
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/tools/virtio/linux/hrtimer.h