opus_parser: make ParseContext the first element in OpusParseContext

ff_parse_close expects priv_data to be the ParseContext directly and
thus doesn't work if it isn't at the beginning of OpusParseContext.
Reviewed-by: Hendrik Leppkes <h.leppkes@gmail.com>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavcodec/opus_parser.c

@@ -31,10 +31,10 @@
 #include "parser.h"
 
 typedef struct OpusParseContext {
+    ParseContext pc;
     OpusContext ctx;
     OpusPacket pkt;
     int extradata_parsed;
-    ParseContext pc;
     int ts_framing;
 } OpusParseContext;