CI: add a comment to the meson build helper

We now have an upstream for it so we can sync changes between projects.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>

.gitlab-ci/meson-build.sh

 #!/usr/bin/env bash
 #
+# This script is sourced from here:
+# https://gitlab.freedesktop.org/whot/meson-helper
+#
 # SPDX-License-Identifier: MIT
 
 set -x