pw-dump: sync on metadata change so that changes are displayed soon

In order to show metadata changes when monitoring, a sync must be
triggered just like it is done for other objects, to make sure that
`dump_objects()` is called some time later and the changed metadata
objects are shown.

Otherwise metadata changes would never be displayed unless there
was a change to a different type of object, whose event handlers
do trigger a sync.

Fixes #4053

src/tools/pw-dump.c

@@ -1231,6 +1231,7 @@ static int metadata_property(void *data,
 		e->changed++;
 	}
 	o->changed++;
+	core_sync(o->data);
 	return 0;
 }