batman-adv: update Makefile copyright years

Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>

net/batman-adv/Makefile

 #
-# Copyright (C) 2007-2012 B.A.T.M.A.N. contributors:
+# Copyright (C) 2007-2013 B.A.T.M.A.N. contributors:
 #
 # Marek Lindner, Simon Wunderlich
 #