classes/copyleft_filter: include AGPL in default included list

classes/copyleft_filter: include AGPL in default included list

If we're including all GPL licenses then we should include AGPL in
the default COPYLEFT_LICENSE_INCLUDE since it has similar terms (and
currently we do have one AGPL-licensed recipe in OE-Core, namely db).

(Note that the default list isn't meant to be a guaranteed complete
safe list - seek legal advice based on what you're distributing if
unsure.)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>