avformat/id3v2: silence a warning when CONFIG_ZLIB is unset.

dlen is only read when CONFIG_ZLIB is set, so mark it as possibly
unused.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/id3v2.c

@@ -726,7 +726,7 @@ static void id3v2_parse(AVIOContext *pb, AVDictionary **metadata,
         int tunsync         = 0;
         int tcomp           = 0;
         int tencr           = 0;
-        unsigned long dlen;
+        unsigned long av_unused dlen;
 
         if (isv34) {
             if (avio_read(pb, tag, 4) < 4)