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>