batman-adv: remove packed from batadv_ogm_packet

As we decreased the struct size from 26 to 24 byte, we can remove
__packed as the compiler will not add any more padding.

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>