sbp2: use __attribute__((packed)) for on-the-wire structures

sbp2: use __attribute__((packed)) for on-the-wire structures

It seems to have worked without the attribute during all the years
just because sizes of all struct members are multiples of 32 bits.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Ben Collins <bcollins@ubuntu.com>