batman-adv: Move definition of atomic_dec_not_zero() into main.h

atomic_dec_not_zero() is very useful and it is currently defined
multiple times. So it is possible to move it in main.h

Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Sven Eckelmann <sven@narfation.org>