Add version to AVClass so we can add to and use fields of AVClass without ABI issues.

Originally committed as revision 22987 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavfilter/avfilter.c

@@ -385,7 +385,9 @@ static const char *filter_name(void *p)
 
 static const AVClass avfilter_class = {
     "AVFilter",
-    filter_name
+    filter_name,
+    NULL,
+    LIBAVUTIL_VERSION_INT,
 };
 
 AVFilterContext *avfilter_open(AVFilter *filter, const char *inst_name)