xfs: fix select in config XFS_ONLINE_SCRUB_STATS

Commit d7a74cad ("xfs: track usage statistics of online fsck")
introduces config XFS_ONLINE_SCRUB_STATS, which selects the non-existing
config FS_DEBUG. It is probably intended to select the existing config
XFS_DEBUG.

Fix the select in config XFS_ONLINE_SCRUB_STATS.

Fixes: d7a74cad ("xfs: track usage statistics of online fsck")
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>