linux: import if_ether.h from kernel-5.0

kernel-4.15's if_ether.h has a bug that the header does not provide
'struct ethhdr'. The bug is introduced by
6926e041a8920c8ec27e4e155efa760aa01551fd (4.15-rc8)
and fixed by da360299b6734135a5f66d7db458dcc7801c826a (4.16-rc3).

This makes systemd built with kernel-4.15 headers.

Fixes #12319.