sched: window-stats: 64-bit type for curr/prev_runnable_sum

Expand rq->curr_runnable_sum and rq->prev_runnable_sum to be 64-bit
counters as otherwise they can easily overflow when a cpu has many
tasks.

Change-Id: I68ab2658ac6a3174ddb395888ecd6bf70ca70473
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>