netem: add SPDX license header

The netem directory contains code to generate tables for netem.
This code came from NISTnet which was public domain.
Add appropriate license tag.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>

netem/maketable.c

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: NIST-PD */
 /*
  * Experimental data  distribution table generator
  * Taken from the uncopyrighted NISTnet code (public domain).

netem/normal.c

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: NIST-PD */
 /*
  * Normal distribution table generator
  * Taken from the uncopyrighted NISTnet code.

netem/pareto.c

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: NIST-PD */
 /*
  * Pareto distribution table generator
  * Taken from the uncopyrighted NISTnet code.

netem/paretonormal.c

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: NIST-PD */
 /*
  * Paretoormal distribution table generator
  *

netem/stats.c

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: NIST-PD */
 /*
  * Experimental data  distribution table generator
  * Taken from the uncopyrighted NISTnet code (public domain).