pcap_io: fill sll when reading pcap

When reading from a pcap in Kuznetsov/netsniff-ng format, we currently do
not fill out sll. Do so so that users can see pkttype and the interface.

Reported-by: Flavio Leitner <fbl@redhat.com>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>