sched: walt: move walt_stats member in cfs_rq under CFS_BANDWIDTH

The walt_stats member (walt_sched_stats structure) defined in cfs_rq
is only needed when CFS_BANDWIDTH feature is enabled. So add this
member under CFS_BANDWIDTH feature in cfs_rq structure.

Change-Id: I9ddfb00da3377a66a18522eaecdb8b0f76517e8e
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>