avfilter: deprecate avfilter_link_get_channels()

And move the channels field to the public section of the struct.

Signed-off-by: James Almer <jamrial@gmail.com>

libavfilter/avfilter.c

@@ -183,10 +183,12 @@ void avfilter_link_free(AVFilterLink **link)
     av_freep(link);
 }
 
+#if FF_API_FILTER_GET_SET
 int avfilter_link_get_channels(AVFilterLink *link)
 {
     return link->channels;
 }
+#endif
 
 void ff_filter_set_ready(AVFilterContext *filter, unsigned priority)
 {