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>

MAINTAINERS

@@ -2796,6 +2796,7 @@ W:	https://www.open-mesh.org/
 B:	https://www.open-mesh.org/projects/batman-adv/issues
 C:	irc://chat.freenode.net/batman
 Q:	https://patchwork.open-mesh.org/project/batman/list/
+T:	git https://git.open-mesh.org/linux-merge.git
 S:	Maintained
 F:	Documentation/ABI/obsolete/sysfs-class-net-batman-adv
 F:	Documentation/ABI/obsolete/sysfs-class-net-mesh