Revert "remove unused variable"

variable is actually used!

This reverts commit 5c7f318bd68ac3256d32c29b8e267e1ba99d09e3.

gtk2_ardour/processor_box.cc

@@ -3030,6 +3030,7 @@ ProcessorBox::add_processor_to_display (boost::weak_ptr<Processor> p)
 		e->set_selectable(false);
 	}
 
+	bool mark_send_visible = false;
 	if (send && _parent_strip) {
 		/* show controls of new sends by default */
 		GUIObjectState& st = _parent_strip->gui_object_state ();