net: make ethernet.h self-contained

[kp]

Summary

Sponsored by: Rubicon Communications, LLC ("Netgate")

[melifaro]

Could you by any chance add the rationale to the diff description?

[kp]

In D33501#757387, @melifaro wrote:

Could you by any chance add the rationale to the diff description?

The point is to ensure that users (both in the kernel and in user space) can include this header without having to know they're supposed to include <sys/types.h> first.
There's a whole series of these, because we have a lot of headers where that doesn't work (and worse cases that just this), and it's often frustrating to go hunting for the required includes.