integrity: Make function integrity_add_key() static

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: 60740accf7 ("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>

security/integrity/digsig.c

@@ -143,7 +143,7 @@ out:
 	return __integrity_init_keyring(id, perm, restriction);
 }
 
-int __init integrity_add_key(const unsigned int id, const void *data,
+static int __init integrity_add_key(const unsigned int id, const void *data,
 				    off_t size, key_perm_t perm)
 {
 	key_ref_t key;