Replace all occurences of PKT_FLAG_KEY with AV_PKT_FLAG_KEY.

Patch by Jean-Daniel Dupas, devlists shadowlab org

Originally committed as revision 22744 to svn://svn.ffmpeg.org/ffmpeg/trunk
diff --git a/libavformat/matroskadec.c b/libavformat/matroskadec.c
index 01c2c7c..8a4ae4e 100644
--- a/libavformat/matroskadec.c
+++ b/libavformat/matroskadec.c
@@ -1547,7 +1547,7 @@
     flags = *data++;
     size -= 3;
     if (is_keyframe == -1)
-        is_keyframe = flags & 0x80 ? PKT_FLAG_KEY : 0;
+        is_keyframe = flags & 0x80 ? AV_PKT_FLAG_KEY : 0;
 
     if (cluster_time != (uint64_t)-1
         && (block_time >= 0 || cluster_time >= -block_time)) {