Use a special struct_ip.h to include <netinet/ip.h> and accoutrements.

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.

scan_engine.cc

@@ -105,6 +105,9 @@
 #include "Target.h"
 #include "targets.h"
 #include "utils.h"
+
+#include "struct_ip.h"
+
 #include <math.h>
 #include <list>
 #include <map>