avcodec/packet_internal: make avpriv_packet_list_* functions use an internal struct

The next pointer is kept at the end for backwards compatability until the
major bump, when it should ideally be moved at the front.

Signed-off-by: James Almer <jamrial@gmail.com>

libavdevice/decklink_common.h

@@ -75,7 +75,7 @@ class decklink_output_callback;
 class decklink_input_callback;
 
 typedef struct AVPacketQueue {
-    AVPacketList *first_pkt, *last_pkt;
+    PacketList *first_pkt, *last_pkt;
     int nb_packets;
     unsigned long long size;
     int abort_request;