netinet: Make ip.h self-contained

In general we are working towards making public headers self-contained.

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

PR: 285924
Reviewed by: emaste
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D49735

(cherry picked from commit 31d3a94bdda4a9ca4c4d7d4e8e8a0ba1b05c7f18)