fix(build): do not use Ethernet_wire

This dependency is not available anymore.

I used the same technique used here

mirage/ethernet@61dfc14

to fix the build.

src/hostnet/hostnet_dns.ml

@@ -257,8 +257,8 @@ struct
  packet creation fn *)
  let frame = Io_page.to_cstruct (Io_page.get 1) in
  let smac = "\000\000\000\000\000\000" in
- Ethernet__Ethernet_wire.set_ethernet_src smac 0 frame;
- Ethernet__Ethernet_wire.set_ethernet_ethertype frame 0x0800;
+ Cstruct.blit_from_string smac 0 frame 6 6;
+ Cstruct.BE.set_uint16 frame 12 0x0800;
  let buf = Cstruct.shift frame Ethernet.Packet.sizeof_ethernet in
  Ipv4_wire.set_ipv4_hlen_version buf ((4 lsl 4) + (5));
  Ipv4_wire.set_ipv4_tos buf 0;
@@ -283,7 +283,7 @@ struct
  let tlen = Cstruct.lenv bufs - Ethernet.Packet.sizeof_ethernet in
  let dmac = String.make 6 '\000' in
  (* Ip.adjust_output_header *)
- Ethernet__Ethernet_wire.set_ethernet_dst dmac 0 frame;
+ Cstruct.blit_from_string dmac 0 frame 0 6;
  let buf =
  Cstruct.sub frame Ethernet.Packet.sizeof_ethernet Ipv4_wire.sizeof_ipv4
  in