batman-adv: Add SPDX license identifier to batman-adv.rst

The "Linux licensing rules" require that also the restructuredText files
are marked with the appropriate SPDX license identifier.
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>

Documentation/networking/batman-adv.rst

+.. SPDX-License-Identifier: GPL-2.0
+
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 batman-adv
 ==========