virtiofs: Fix old-style declaration

There expect the 'static' keyword to come first in a declaration, and we
get warnings like this with "make W=1":

fs/fuse/virtio_fs.c:687:1: warning: 'static' is not at beginning of declaration [-Wold-style-declaration]
fs/fuse/virtio_fs.c:692:1: warning: 'static' is not at beginning of declaration [-Wold-style-declaration]
fs/fuse/virtio_fs.c:1029:1: warning: 'static' is not at beginning of declaration [-Wold-style-declaration]

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>