pkt: make knot_pkt_qname() return const*

This commit makes knot_pkt_qname() return a const pointer to indicate to
callers that the contents (the QNAME in the wire format buffer) should
not be modified.

src/libknot/packet/pkt.h

@@ -179,7 +179,7 @@ static inline uint16_t knot_pkt_question_size(const knot_pkt_t *pkt)
 	return pkt->qname_size + 2 * sizeof(uint16_t);
 }
 
-static inline knot_dname_t *knot_pkt_qname(const knot_pkt_t *pkt)
+static inline const knot_dname_t *knot_pkt_qname(const knot_pkt_t *pkt)
 {
 	if (pkt == NULL || pkt->qname_size == 0) {
 		return NULL;