sched/cputime: Add dummy cputime_adjust() implementation for CONFIG_VIRT_CPU_ACCOUNTING_NATIVE

cfb766da ("sched/cputime: Expose cputime_adjust()") made
cputime_adjust() public for cgroup basic cpu stat support; however,
the commit forgot to add a dummy implementaiton for
CONFIG_VIRT_CPU_ACCOUNTING_NATIVE leading to compiler errors on some
s390 configurations.

Fix it by adding the missing dummy implementation.

Reported-by: “kbuild-all@01.org” <kbuild-all@01.org>
Fixes: cfb766da ("sched/cputime: Expose cputime_adjust()")
Signed-off-by: Tejun Heo <tj@kernel.org>