drivers: virtio: Mark function virtballoon_migratepage() as static in virtio_balloon.c

Mark the function virtballoon_migratepage() as static in
virtio_balloon.c because it is not used outside this file.

This eliminates the following warning in virtio_balloon.c:
drivers/virtio/virtio_balloon.c:372:5: warning: no previous prototype for ‘virtballoon_migratepage’ [-Wmissing-prototypes]
Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/virtio/virtio_balloon.c

@@ -369,7 +369,7 @@ static const struct address_space_operations virtio_balloon_aops;
  * This function preforms the balloon page migration task.
  * Called through balloon_mapping->a_ops->migratepage
  */
-int virtballoon_migratepage(struct address_space *mapping,
+static int virtballoon_migratepage(struct address_space *mapping,
 		struct page *newpage, struct page *page, enum migrate_mode mode)
 {
 	struct balloon_dev_info *vb_dev_info = balloon_page_device(page);