aggregator: fix flow-return boolean return type mismatch

Not that it matters, since we don't check the return value
anyway. Unclear why the aggregator pad flush function should
have a return value at all really, and perhaps it should be
called reset anyway. Spotted by dv on irc.

libs/gst/base/gstaggregator.c

@@ -295,7 +295,7 @@ gst_aggregator_pad_flush (GstAggregatorPad * aggpad, GstAggregator * agg)
   PAD_UNLOCK (aggpad);
 
   if (klass->flush)
-    return klass->flush (aggpad, agg);
+    return (klass->flush (aggpad, agg) == GST_FLOW_OK);
 
   return TRUE;
 }