batman-adv: Add license header to Kconfig

The last remaining file without license notice and/or SPDX license
identifier under net/batman-adv/ is the Kconfig. It should have been
licensed under the same conditions as the rest of batman-adv and the
Makefile which uses the CONFIG_* variables from Kconfig.
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>

net/batman-adv/Kconfig

+# SPDX-License-Identifier: GPL-2.0
+# Copyright (C) 2007-2017  B.A.T.M.A.N. contributors:
+#
+# Marek Lindner, Simon Wunderlich
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of version 2 of the GNU General Public
+# License as published by the Free Software Foundation.
+#
+# This program is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+# General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, see <http://www.gnu.org/licenses/>.
+
 #
 # B.A.T.M.A.N meshing protocol
 #