xfs: Remove obsolete declaration of xfs_buf_get_empty

This function has been removed ever since at least 3.12-era. No need to
keep its declaration in the header so nuke it.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>