lavfi: Move new field to the end of AVFilterLink

Even though this is not part of the public API, some external
applications access fields after it, thus breaking after updating from
ffmpeg 3.0 or earlier.
Since it is not public, it can be freely moved to the end to avoid
that problem in the future.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>

libavfilter/avfilter.h

@@ -473,12 +473,6 @@ struct AVFilterLink {
      */
     AVRational frame_rate;
 
-    /**
-     * For hwaccel pixel formats, this should be a reference to the
-     * AVHWFramesContext describing the frames.
-     */
-    AVBufferRef *hw_frames_ctx;
-
     /**
      * Buffer partially filled with samples to achieve a fixed/minimum size.
      */
@@ -550,6 +544,12 @@ struct AVFilterLink {
      * cleared when a frame is filtered.
      */
     int frame_wanted_out;
+
+    /**
+     * For hwaccel pixel formats, this should be a reference to the
+     * AVHWFramesContext describing the frames.
+     */
+    AVBufferRef *hw_frames_ctx;
 };
 
 /**