sched: Replace zero-length array with flexible-array

In the following commit:

  04f5c362: ("sched/fair: Replace zero-length array with flexible-array")

a zero-length array cpumask[0] has been replaced with cpumask[].
But there is still a cpumask[0] in 'struct sched_group_capacity'
which was missed.

The point of using [] instead of [0] is that with [] the compiler will
generate a build warning if it isn't the last member of a struct.

[ mingo: Rewrote the changelog. ]

Signed-off-by: zhuguangqing <zhuguangqing@xiaomi.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20201014140220.11384-1-zhuguangqing83@gmail.com