Define DLT_LINUX_SLL2

As required in order to be able to dump both interface index and packet direction. Header structure copied from the current form of 'struct sockaddr_sll' on Linux, a very early version of which was used by the original LINKTYPE_LINUX_SLL but Linux's actual structure has changed much since then.

Required for https://github.com/the-tcpdump-group/libpcap/issues/127