MINOR: dns: comments in types/dns.h about structures endianness

To avoid issues when porting code to some architecture, we need to know
the endianess the structures are currently used.
This patch simply had a short notice before those structures to report
endianess and ease contributor's job.
diff --git a/include/types/dns.h b/include/types/dns.h
index 5d6b5a1..b339249 100644
--- a/include/types/dns.h
+++ b/include/types/dns.h
@@ -90,11 +90,13 @@
 } __attribute__ ((packed));
 
 /* short structure to describe a DNS question */
+/* NOTE: big endian structure */
 struct dns_question {
 	unsigned short	qtype;		/* question type */
 	unsigned short	qclass;		/* query class */
 };
 
+/* NOTE: big endian structure */
 struct dns_query_item {
 	struct list list;
 	char name[DNS_MAX_NAME_SIZE];		/* query name */
@@ -102,6 +104,7 @@
 	unsigned short class;			/* query class */
 };
 
+/* NOTE: big endian structure */
 struct dns_answer_item {
 	struct list list;
 	char *name;				/* answer name