aout: set discontinuity after a flush

This commit doesn't have any impact on pulse, since a flush will cause the next
play to be differed.

This commit fixes the "playback way too early" with other modules, like asla,
when flushing with an audio delay.

src/audio_output/dec.c

@@ -441,4 +441,5 @@ void aout_DecFlush (audio_output_t *aout, bool wait)
 
         aout->flush(aout, wait);
     }
+    owner->sync.discontinuity = true;
 }