security/suricata: Add PPPoE support

Backport upstream Pull Request #1416 [1] adding support for
LINKTYPE_NULL.

[1] https://github.com/inliniac/suricata/pull/1416
[2] https://redmine.openinfosecfoundation.org/issues/1445

Submitted by: Bill Meeks <bmeeks8 bellsouth net> (via gnn)