avcodec/ac3: move new field to the end of AC3HeaderInfo

This structure is used in the interface between libs and thus cannot have
fields added in the middle without major bump
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/ac3.h

@@ -142,7 +142,9 @@ typedef struct AC3HeaderInfo {
     int surround_mix_level;                 ///< Surround mix level index
     uint16_t channel_map;
     int num_blocks;                         ///< number of audio blocks
+#if AV_HAVE_INCOMPATIBLE_LIBAV_ABI
     int dolby_surround_mode;
+#endif
     /** @} */
 
     /** @name Derived values
@@ -155,6 +157,9 @@ typedef struct AC3HeaderInfo {
     uint16_t frame_size;
     uint64_t channel_layout;
     /** @} */
+#if !AV_HAVE_INCOMPATIBLE_LIBAV_ABI
+    int dolby_surround_mode;
+#endif
 } AC3HeaderInfo;
 
 typedef enum {