binary compatibility fix (workaround for r.147, r.148)

Originally committed as revision 1702 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/avcodec.h

@@ -797,6 +797,10 @@ typedef struct AVCodecContext {
      */
     float dark_masking;
     
+    
+    /* for binary compatibility */
+    int unused;
+    
     /**
      * idct algorithm, see FF_IDCT_* below.
      * - encoding: set by user