MAINTAINERS: Add B(ugtracker) field for batman-adv

While it is acceptable to discuss problems on the mailing list, it is
easier to track bugs in the official bugtracker. Registration is required
to get access to the submission form.

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