MAINTAINERS: Add T(ree) field for batman-adv

The linux-merge.git repository on git.open-mesh.org is used since 8 years
to send PRs for net.git and net-next.git. It is time to officially specify
list it as SCM tree.

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