batman-adv: Fix kernel-doc parsing of main structs

kernel-doc is not able to skip an #ifdef between the kernel documentation
block and the start of the struct. Moving the #ifdef before the kernel doc
block avoids this problem

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <a@unstable.cc>