modify the way to pass parameters to av_metadata_set()

This improves code readability and this avoid warnings about discarding
qualifiers from pointer target type.

Originally committed as revision 16952 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/avidec.c

@@ -229,7 +229,7 @@ static int avi_read_tag(AVFormatContext *s, const char *key, unsigned int size)
     get_strz(pb, value, sizeof(value));
     url_fseek(pb, i+size, SEEK_SET);
 
-    return av_metadata_set(&s->metadata, (const AVMetadataTag){key, value});
+    return av_metadata_set(&s->metadata, key, value);
 }
 
 static int avi_read_header(AVFormatContext *s, AVFormatParameters *ap)