Fix xfs debug build breakage by pushing xfs_error.h after

xfs_mount.h, which it depends on.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Alex Elder <aelder@sgi.com>
Signed-off-by: Felix Blyakher <felixb@sgi.com>