audit: make is_audit_feature_set() static

Currently nobody use is_audit_feature_set() outside this file, so make
it static.

Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

kernel/audit.c

@@ -1100,7 +1100,7 @@ static inline void audit_log_user_recv_msg(struct audit_buffer **ab,
 	audit_log_common_recv_msg(NULL, ab, msg_type);
 }
 
-int is_audit_feature_set(int i)
+static int is_audit_feature_set(int i)
 {
 	return af.features & AUDIT_FEATURE_TO_MASK(i);
 }