batman-adv: compat: Fix skb_vlan_eth_hdr conflict in stable kernels

The newest Linux stable kernel releases:

* v5.10.205
* v5.15.144
* v6.1.69

received a backported version of commit 1f5020acb33f ("net: vlan: introduce
skb_vlan_eth_hdr()"). batman-adv must therefore not provide this function
any longer for these kernel versions.

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