lavf: export flush_packet_queue() as mythtv_flush_packet_queue()

FFmpeg 5.0 renames and exports this as ff_flush_packet_queue().

This change is intended to make transitioning to FFmpeg 5.0 easier.

Technically, this should be in libavformat/internal.h, but, since it is currently
unmodified, libavformat/avformat.h will work for now.  (MythTV does not directly use
this function.)

mythtv/external/FFmpeg/libavformat/avformat.h

@@ -3022,7 +3022,7 @@ AVRational av_stream_get_codec_timebase(const AVStream *st);
 /* MythTV changes */
 void av_estimate_timings(AVFormatContext *ic, int64_t old_offset);
 void av_remove_stream(AVFormatContext *s, int id, int remove_ts);
-void flush_packet_queue(AVFormatContext *s);
+void mythtv_flush_packet_queue(AVFormatContext *s);
 /* End MythTV changes */
 
 /**

mythtv/external/FFmpeg/libavformat/mpegts-mythtv.c

@@ -151,7 +151,7 @@ static void av_mpegts_remove_stream(AVFormatContext *s, int id) {
     if (changes)
     {
         // flush queued packets after a stream change (might need to make smarter)
-        flush_packet_queue(s);
+        mythtv_flush_packet_queue(s);
 
         /* renumber the streams */
         av_log(NULL, AV_LOG_DEBUG, "av_mpegts_remove_stream: renumbering streams\n");

mythtv/external/FFmpeg/libavformat/utils.c

@@ -1853,7 +1853,7 @@ int av_read_frame(AVFormatContext *s, AVPacket *pkt)
 }
 
 /* XXX: suppress the packet queue */
-void flush_packet_queue(AVFormatContext *s)
+static void flush_packet_queue(AVFormatContext *s)
 {
     if (!s->internal)
         return;
@@ -6002,3 +6002,9 @@ void av_remove_stream(AVFormatContext *s, int id, int /* remove_ts */) {
             s->streams[i]->index=i;
     }
 }
+
+// temporary export for mpegts-mythtv.c; FFmpeg 5.0 exports it as ff_flush_packet_queue
+void mythtv_flush_packet_queue(AVFormatContext *s)
+{
+    flush_packet_queue(s);
+}