integrity: Make function integrity_add_key() static

mainline inclusion
from mainline-v5.12-rc1
commit f6692213
category: bugfix
bugzilla: 187303, https://gitee.com/openeuler/kernel/issues/I5ZXPC
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f6692213b5045dc461ce0858fb18cf46f328c202



-------------------------------

The sparse tool complains as follows:

security/integrity/digsig.c:146:12: warning:
 symbol 'integrity_add_key' was not declared. Should it be static?

This function is not used outside of digsig.c, so this
commit marks it static.

Reported-by: Hulk Robot <hulkci@huawei.com>
Fixes: 60740acc ("integrity: Load certs to the platform keyring")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Nayna Jain <nayna@linux.ibm.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>
Signed-off-by: Cai Xinchen <caixinchen1@huawei.com>
Reviewed-by: Wang Weiyang <wangweiyang2@huawei.com>
Reviewed-by: GUO Zihua <guozihua@huawei.com>
Reviewed-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>