audiobasesink: Post latency message on the bus after set_caps()

Any latency query before this will not get the correct latency so a new
latency query should be triggered once the audio sink know its own latency.

Without this the initial latency query from the pipeline arrives too early
sometimes and the resulting latency is too short.

https://bugzilla.gnome.org/show_bug.cgi?id=758911

gst-libs/gst/audio/gstaudiobasesink.c

@@ -1006,6 +1006,9 @@ gst_audio_base_sink_setcaps (GstBaseSink * bsink, GstCaps * caps)
 
   gst_audio_ring_buffer_debug_spec_buff (spec);
 
+  gst_element_post_message (GST_ELEMENT_CAST (bsink),
+      gst_message_new_latency (GST_OBJECT (bsink)));
+
   return TRUE;
 
   /* ERRORS */