sched: fix compilation error in task_fits_capacity for !SCHED_WALT

When SCHED_WALT is disabled, max_capacity won't be available and
results in compilation error. Fix this by defining the max_capcity
locally.

Change-Id: I211a255adb574bdea9db1ff2f5797f849050df73
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>