doc: add SPDX license tag header to Intel performance guide

[ upstream commit a0abf70 ]

This document never had any license or copyright on this file, add one.

Fixes: b932ebc ("doc: add NIC performance guide on Linux IA")

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Hemant Agrawal <hemant.agrawal@nxp.com>
Acked-by: John McNamara <john.mcnamara@intel.com>

doc/guides/linux_gsg/nic_perf_intel_platform.rst

@@ -1,3 +1,6 @@
+..  SPDX-License-Identifier: BSD-3-Clause
+    Copyright(c) 2015 Intel Corporation.
+
 How to get best performance with NICs on Intel platforms
 ========================================================