xfs: make xfs_iomap_page_ops static

Shut up the sparse warnings about this variable that isn't referenced
anywhere else.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>

fs/xfs/xfs_iomap.c

@@ -83,7 +83,7 @@ xfs_iomap_valid(
 	return true;
 }
 
-const struct iomap_page_ops xfs_iomap_page_ops = {
+static const struct iomap_page_ops xfs_iomap_page_ops = {
 	.iomap_valid		= xfs_iomap_valid,
 };