doc: add SPDX license tag header to meson guide

The build-sdk-meson.rst file originates from the short plain-text meson
instructions added in 2018. Add SPDX tag and copyright notice based on the
original commit.

Fixes: 9c3adc2 ("doc: add instructions on build using meson")
Cc: stable@dpdk.org

Reported-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>

Author: bruce-richardson

doc/guides/prog_guide/build-sdk-meson.rst

@@ -1,3 +1,6 @@
+..  SPDX-License-Identifier: BSD-3-Clause
+    Copyright(c) 2018 Intel Corporation.
+
 Installing DPDK Using the meson build system
 ============================================