ANDROID: sched/walt: fix build failure if FAIR_GROUP_SCHED=n

ANDROID: sched/walt: fix build failure if FAIR_GROUP_SCHED=n

Fix SCHED_WALT dependency on FAIR_GROUP_SCHED otherwise we run
into following build failure:

  CC      kernel/sched/walt.o
kernel/sched/walt.c: In function 'walt_inc_cfs_cumulative_runnable_avg':
kernel/sched/walt.c:148:8: 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:154:8: error: 'struct cfs_rq' has no member named 'cumulative_runnable_avg'
  cfs_rq->cumulative_runnable_avg -= p->ravg.demand;
        ^

Reported-at: https://bugs.linaro.org/show_bug.cgi?id=2793
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>