xfs: mark xlog_verify_dest_ptr STATIC

We already did it in the forward declaration, but not for the function
body itself.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>