if_ether.h: add B.A.T.M.A.N.-Advanced Ethertype

Add Ethertype 0x4305 (not an officially registered id).
This Ethertype is used by every frame generated by B.A.T.M.A.N.-Advanced. Its
definition is currently batman-adv local only and since it is not officially
registered it is better to make its definition kernel-wide so that we avoid
collisions given by future unofficial uses of the same Ethertype.

Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: David S. Miller <davem@davemloft.net>