scripts: mandate that new files have SPDX-License-Identifier

scripts: mandate that new files have SPDX-License-Identifier

Going forward we want all newly created source files to have an
SPDX-License-Identifier tag present.

Initially mandate this for C, Python, Perl, Shell source files,
as well as JSON (QAPI) and Makefiles, while encouraging users
to consider it for other file types.

Reviewed-by: Brian Cain <bcain@quicinc.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>