ima: Make arch_policy_entry static

Fix sparse warning:

security/integrity/ima/ima_policy.c:202:23: warning:
 symbol 'arch_policy_entry' was not declared. Should it be static?

Fixes: 61917062

 ("ima: add support for arch specific policies")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Cc: stable@vger.kernel.org (linux-5.0)
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>