add licensing information

Signed-off-by: Alessandro Comodi <acomodi@antmicro.com>

tests/mux_class/mux_class.sim.v

diff --git a/tests/mux_class/mux_class.sim.v b/tests/mux_class/mux_class.sim.v
index 92aede4..497c953 100644
--- a/tests/mux_class/mux_class.sim.v
+++ b/tests/mux_class/mux_class.sim.v
@@ -1,4 +1,12 @@
-`default_nettype none
+/*
+ * Copyright (C) 2020  The SymbiFlow Authors.
+ *
+ * Use of this source code is governed by a ISC-style
+ * license that can be found in the LICENSE file or at
+ * https://opensource.org/licenses/ISC
+ *
+ * SPDX-License-Identifier:	ISC
+ */
 
 (* CLASS="mux" *)
 (* MODEL_NAME="MUX_MODEL_NAME" *)