sys/fuchsia: add zircon-ethernet to fidlgen libs
sys/fuchsia: add zircon-ethernet to fidlgen libs

This commits adds "zircon-ethernet" to the list of fidl libraries that
fidlgen would look at. This is required because recently, a change in
fuchsia[0] added a new method to the netstack interface, that allows for
adding an EthernetInterface. One of the parameters is a
zircon.ethernet.Device, so we need to import that type for fidlgen to be
able to understand fidl_net-stack.txt.

I tested this by running:

```shell
$ make extract TARGETOS=fuchsia SOURCEDIR=${FUCHSIADIR}
$ make generate
```

[0]: https://fuchsia-review.googlesource.com/c/zircon/+/220580
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