apparmor: document capability.c:profile_capable ad ptr not being NULL

The profile_capabile function takes a struct apparmor_audit_data *ad,
which is documented as possibly being NULL. However, the single place that
calls this function never passes it a NULL ad. If we were ever to call
profile_capable with a NULL ad elsewhere, we would need to rework the
function, as its very first use of ad is to dereference ad->class without
checking if ad is NULL.

Thus, document profile_capable's ad parameter as not accepting NULL.

Signed-off-by: Ryan Lee <ryan.lee@canonical.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>