lavfi: deprecate avfilter_graph_add_filter().

Since this function adds a standalone filter to a filter graph and we do
not support creating such filters, there is no reason for this function
to exist.

libavfilter/avfiltergraph.c

@@ -58,6 +58,7 @@ void avfilter_graph_free(AVFilterGraph **graph)
     av_freep(graph);
 }
 
+#if FF_API_AVFILTER_OPEN
 int avfilter_graph_add_filter(AVFilterGraph *graph, AVFilterContext *filter)
 {
     AVFilterContext **filters = av_realloc(graph->filters,
@@ -74,6 +75,7 @@ int avfilter_graph_add_filter(AVFilterGraph *graph, AVFilterContext *filter)
 
     return 0;
 }
+#endif
 
 int avfilter_graph_create_filter(AVFilterContext **filt_ctx, AVFilter *filt,
                                  const char *name, const char *args, void *opaque,