apparmor: Add __init annotation to aa_{setup/teardown}_dfa_engine()

The aa_setup_dfa_engine() and aa_teardown_dfa_engine() is only called in
apparmor_init(), so let us add __init annotation to them.

Fixes: 11c236b8 ("apparmor: add a default null dfa")
Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>