netinet: Make ip.h self-contained

[markj]

Summary

cdefs.h is included for __packed; just assume that types.h includes
cdefs.h as that's a very common assumption.

PR: 285924

[emaste]

IMO the commit message could use one more sentence introducing the justification (which was reasonably explained in either the PR or the upstream report IIRC) but LGTM.

[emaste]

Also we have some commentary in style(9) about kernel headers visible to userland applications:

In header files visible to userland applications, prototypes that are
visible must use either “protected” names (ones beginning with an
underscore) or no names with the types.  It is preferable to use
protected names.  E.g., use:

We ought to make a note of our (evolving) approach wrt self-contained headers there. I can pick that up if nobody else wants to.

[markj]

In D49735#1133933, @emaste wrote:

We ought to make a note of our (evolving) approach wrt self-contained headers there. I can pick that up if nobody else wants to.

Since you volunteered... :)