MINOR: dns: New DNS response analysis code: DNS_RESP_TRUNCATED

This patch introduces a new internal response state about the analysis
of a DNS response received by a server.
It is dedicated to report to above layer that the response is
'truncated'.
diff --git a/include/types/dns.h b/include/types/dns.h
index ba35c71..3ec3492 100644
--- a/include/types/dns.h
+++ b/include/types/dns.h
@@ -198,6 +198,7 @@
 	DNS_RESP_WRONG_NAME,		/* response does not match query name */
 	DNS_RESP_CNAME_ERROR,		/* error when resolving a CNAME in an atomic response */
 	DNS_RESP_TIMEOUT,		/* DNS server has not answered in time */
+	DNS_RESP_TRUNCATED,		/* DNS response is truncated */
 };
 
 /* return codes after searching an IP in a DNS response buffer, using a family preference */