Fix the following sparse warning:

arch/arm64/kernel/smp.c:68:5: warning: symbol 'cpus_stuck_in_kernel'
was not declared. Should it be static?
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zou Wei <zou_wei@huawei.com>
Link: https://lore.kernel.org/r/1587623606-96698-1-git-send-email-zou_wei@huawei.comSigned-off-by: Will Deacon <will@kernel.org>