batman-adv: Move batadv_sum_counter to soft-interface.c

The function batadv_sum_counter is only used in soft-interface.c and has no
special relevance for main.h.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>