sched: Fix compilation issues with schedutil for !SCHED_WALT

sched_ravg_window and sysctl_sched_use_walt_cpu_util are WALT specific
tunables. These are used outside !SCHED_WALT in schedutil. Add stubs
for these to fix compilation with WALT disabled.

Change-Id: I3f61cf06a857d52da5eda1d51701ad7cc21598d3
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>