batman-adv: Fix search for cfg80211 module symbols

batman-adv: Fix search for cfg80211 module symbols

The Module.symvers with the exported symbols of all other kernel modules in
OpenWrt is usually placed in the main source directory of the package. But
the actual sources for batman-adv are found in net/batman-adv. OpenWrt must
therefore be informed to move it to this subdirectory or otherwise modpost
will fail to find the symbols

  WARNING: "cfg80211_get_station" [.../batman-adv-2018.3/net/batman-adv/batman-adv.ko] undefined!

The dependency will then be missing in the .modinfo depends= option and
thus it the kernel module loader will not correctly load the cfg80211.ko
during bootup.

Signed-off-by: Sven Eckelmann <sven@narfation.org>