batman-adv: Add Sven to MAINTAINERS file

Sven is taking care of tracking our patches and merging most of them in
our tree. Let's add him to the MAINTAINERS file so he will get all
patch e-mails.

Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
Acked-by: Antonio Quartulli <a@unstable.cc>