doc: add SPDX license tag header to meson guide

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: 9c3adc289c5e ("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>