batman-adv: add build check macros for packet member offset

Since we removed the __packed from most of the packets, we should
make sure that the offset generated by the compiler are correct for
sent/received data.

Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>