pdu.[hc]: make argument of coap_new_pdu() const

As the session object passed to coap_new_pdu() is not modified
it should be const.

include/coap2/pdu.h

@@ -385,7 +385,7 @@ void coap_pdu_clear(coap_pdu_t *pdu, size_t size);
 /**
  * Creates a new CoAP PDU.
  */
-coap_pdu_t *coap_new_pdu(struct coap_session_t *session);
+coap_pdu_t *coap_new_pdu(const struct coap_session_t *session);
 
 /**
  * Dispose of an CoAP PDU and frees associated storage.

src/pdu.c

@@ -128,7 +128,7 @@ coap_pdu_init(uint8_t type, uint8_t code, uint16_t tid, size_t size) {
 }
 
 coap_pdu_t *
-coap_new_pdu(struct coap_session_t *session) {
+coap_new_pdu(const struct coap_session_t *session) {
   coap_pdu_t *pdu = coap_pdu_init(0, 0, 0, coap_session_max_pdu_size(session));
 #ifndef NDEBUG
   if (!pdu)