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>