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)

sys/netinet/ip.h

@@ -35,7 +35,8 @@
 #ifndef _NETINET_IP_H_
 #define	_NETINET_IP_H_
 
-#include <sys/cdefs.h>
+#include <sys/types.h>
+#include <netinet/in.h>
 
 /*
  * Definitions for internet protocol version 4.