ANDROID: sched/walt: Fix compilation issue for x86_64

Below compilation errors observed when SCHED_WALT enabled and
FAIR_GROUP_SCHED is disabled, fix it.

  CC      kernel/sched/walt.o
kernel/sched/walt.c: In function .walt_inc_cfs_cumulative_runnable_avg.:
kernel/sched/walt.c:157:8: error: .struct cfs_rq. has no member named .cumulative_runnable_avg.
  cfs_rq->cumulative_runnable_avg += p->ravg.demand;
        ^
kernel/sched/walt.c: In function .walt_dec_cfs_cumulative_runnable_avg.:
kernel/sched/walt.c:163:8: error: .struct cfs_rq. has no member named .cumulative_runnable_avg.
  cfs_rq->cumulative_runnable_avg -= p->ravg.demand;
        ^
make[2]: *** [kernel/sched/walt.o] Error 1
make[1]: *** [kernel/sched] Error 2
make: *** [kernel] Error 2

Change-Id: I52d506aa5494f4c351f19e6bb2f1a75c8333f321
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>
(cherry-picked for android-4.14)
Signed-off-by: Chris Redpath <chris.redpath@arm.com>