lavc/avpacket: Make pkt parameter of av_packet_get_side_data() const.

Reflects the actual code and silences a gcc warning:
libavcodec/utils.c:2102:36: warning: passing argument 1 of 'av_packet_get_side_data' discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]

libavcodec/avcodec.h

@@ -4572,7 +4572,7 @@ int av_packet_shrink_side_data(AVPacket *pkt, enum AVPacketSideDataType type,
  * @param size pointer for side information size to store (optional)
  * @return pointer to data if present or NULL otherwise
  */
-uint8_t* av_packet_get_side_data(AVPacket *pkt, enum AVPacketSideDataType type,
+uint8_t* av_packet_get_side_data(const AVPacket *pkt, enum AVPacketSideDataType type,
                                  int *size);
 
 int av_packet_merge_side_data(AVPacket *pkt);

libavcodec/avpacket.c

@@ -336,7 +336,7 @@ uint8_t *av_packet_new_side_data(AVPacket *pkt, enum AVPacketSideDataType type,
     return data;
 }
 
-uint8_t *av_packet_get_side_data(AVPacket *pkt, enum AVPacketSideDataType type,
+uint8_t *av_packet_get_side_data(const AVPacket *pkt, enum AVPacketSideDataType type,
                                  int *size)
 {
     int i;