batman-adv: Add license header to Kconfig

The last remaining file without license notice and/or SPDX license
identifier under net/batman-adv/ is the Kconfig. It should have been
licensed under the same conditions as the rest of batman-adv and the
Makefile which uses the CONFIG_* variables from Kconfig.

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