cgroup: mark @cgrp __maybe_unused in cpu_stat_show()

The local variable @cgrp isn't used if !CONFIG_CGROUP_SCHED.  Mark the
variable with __maybe_unused to avoid a compile warning.

Reported-by: "kbuild-all@01.org" <kbuild-all@01.org>
Signed-off-by: Tejun Heo <tj@kernel.org>