lavfi/afade: add timeline support

For correct precision one may need to use asetnsamples.
Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavfilter/af_afade.c

@@ -297,4 +297,5 @@ AVFilter avfilter_af_afade = {
     .inputs        = avfilter_af_afade_inputs,
     .outputs       = avfilter_af_afade_outputs,
     .priv_class    = &afade_class,
+    .flags         = AVFILTER_FLAG_SUPPORT_TIMELINE_GENERIC,
 };