Warning fix: Make ogg_stream.codec const

This matches the return value of ogg_find_codec and fixes
"libavformat/oggdec.c:333: warning: assignment discards qualifiers from
pointer target type"

Patch by Daniel Verkamp (daniel - drv . nu)

Originally committed as revision 18769 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/oggdec.h

@@ -53,7 +53,7 @@ struct ogg_stream {
     uint32_t seq;
     uint64_t granule, lastgp;
     int flags;
-    struct ogg_codec *codec;
+    const struct ogg_codec *codec;
     int header;
     int nsegs, segp;
     uint8_t segments[255];