MAINTAINERS: Add C(hat) field for batman-adv

The #batman channel on freenode was created to discuss various B.A.T.M.A.N.
related topics (like batman-adv).
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>

MAINTAINERS

@@ -2794,6 +2794,7 @@ M:	Antonio Quartulli <a@unstable.cc>
 L:	b.a.t.m.a.n@lists.open-mesh.org (moderated for non-subscribers)
 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/
 S:	Maintained
 F:	Documentation/ABI/obsolete/sysfs-class-net-batman-adv