xT: improve MSYS2 build

The static Qt 6 package is unusable.

xT/xT.cpp

@@ -1421,6 +1421,7 @@ static void
 build_main_window()
 {
 	g.wMain = new QMainWindow;
+	refresh_icon();
 
 	auto central = new QWidget(g.wMain);
 	auto vbox = new QVBoxLayout(central);