sched: walt: Use sched_avg for WALT only

sched_avg stats enabled for SMP, but it is used only in WALT.
So move sched_avg under WALT.

Change-Id: I19b558833e12b80f02fea27c9a9fc8b7630d8689
Signed-off-by: Lingutla Chandrasekhar <clingutla@codeaurora.org>