This was failing to compile to platforms where HAVE_STRUCT_IP is true
but HAVE_STRUCT_ICMP is false, because struct ip was not being defined
(by a system header) before struct icmp.
The comment in struct_ip.h explains the reasoning for this. The AIX C library
uses #defines that change the names of members of struct ip, and conflict with
some existing code. (Notably struct ip_hdr in libdnet and IPv4Header::h in
libnetutil.) We can still use the AIX files if we include <netinet/ip.h> after
this other code has been preprocessed. That's hard to enforce when
<netinet/ip.h> is included from another header file; this new file allows
including it always late, and only where needed.
