Build changes for GNU/Hurd.

--- a/netdissect-stdinc.h
+++ b/netdissect-stdinc.h
@@ -228,7 +228,11 @@
 
 #include <unistd.h>
 #include <netdb.h>
+#ifndef __GNU__
 #include <sys/param.h>
+#else
+#include <rpc/types.h>
+#endif
 #include <sys/types.h>			/* concession to AIX */
 #include <sys/time.h>
 #include <sys/socket.h>