batman-adv: Kconfig, Add missing DEBUG_FS dependency

BATMAN_ADV_DEBUG is using debugfs files for the debugging log. So it
depends on DEBUG_FS which is missing as dependency in the Kconfig file.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>