include/uapi/linux/virtio_balloon.h: include linux/virtio_types.h

Fixes userspace compilation error:

error: unknown type name ‘__virtio16’
  __virtio16 tag;
Signed-off-by: Mikko Rapeli <mikko.rapeli@iki.fi>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

include/uapi/linux/virtio_balloon.h

@@ -26,6 +26,7 @@
  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE. */
 #include <linux/types.h>
+#include <linux/virtio_types.h>
 #include <linux/virtio_ids.h>
 #include <linux/virtio_config.h>