virtio-balloon: make virtballoon_free_page_report() static

Fix the following sparse warning:

drivers/virtio/virtio_balloon.c:168:5: warning: symbol
'virtballoon_free_page_report' was not declared. Should it be static?
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Link: https://lore.kernel.org/r/20200409085047.45483-1-yanaijie@huawei.comSigned-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/virtio/virtio_balloon.c

@@ -165,7 +165,7 @@ static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq)
 
 }
 
-int virtballoon_free_page_report(struct page_reporting_dev_info *pr_dev_info,
+static int virtballoon_free_page_report(struct page_reporting_dev_info *pr_dev_info,
 				   struct scatterlist *sg, unsigned int nents)
 {
 	struct virtio_balloon *vb =