wpantund: Whitelist CVE entry where issue is already fixed

The CVE issue CVE-2020-8916 has been fixed in the upstream repo for a
long time and our recipe has this commit already included.

https://cve.mitre.org/cgi-bin/cvename.cgi?name=2020-8916



Mark the CVE as such.

Signed-off-by: Stefan Schmidt <stefan.schmidt@huawei.com>