package/batman-adv: fix license

batman_adv.h is licensed under MIT

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/batman-adv/batman-adv.mk

@@ -6,7 +6,7 @@
 
 BATMAN_ADV_VERSION = 2017.0.1
 BATMAN_ADV_SITE = https://downloads.open-mesh.org/batman/stable/sources/batman-adv
-BATMAN_ADV_LICENSE = GPL-2.0
+BATMAN_ADV_LICENSE = GPL-2.0, MIT (batman_adv.h)
 
 # Bridge Loop Avoidance, Distributed Arp Table are always enabled
 BATMAN_ADV_CFLAGS = \