un-static xfs_read_agf

CONFIG_XFS_DEBUG builds still need xfs_read_agf to be
non-static, oops.

Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Reviewed-by: Felix Blyakher <felixb@sgi.com>
Signed-off-by: Felix Blyakher <felixb@sgi.com>