meson: Enable -Wvla

QEMU has historically used variable length arrays only very rarely.
Variable length arrays are a potential security issue where an
on-stack dynamic allocation isn't correctly size-checked, especially
when the size comes from the guest.  (An example problem of this kind
from the past is CVE-2021-3527).  Forbidding them entirely is a
defensive measure against further bugs of this kind.

Enable -Wvla to prevent any new uses from sneaking into the codebase.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-ID: <20240125173211.1786196-3-peter.maydell@linaro.org>
[thuth: rebased to current master branch]
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20240221162636.173136-4-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>