avformat/hcom: Tell the compiler about set but not read variables

This avoids 3 warnings
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/hcom.c

@@ -38,7 +38,7 @@ static int hcom_probe(const AVProbeData *p)
 static int hcom_read_header(AVFormatContext *s)
 {
     AVStream *st;
-    unsigned data_size, rsrc_size, huffcount;
+    av_unused unsigned data_size, rsrc_size, huffcount;
     unsigned compresstype, divisor;
     unsigned dict_entries;
     int ret;