In public headers, we want to avoid things that are not standard C++,
including features that GCC and Clang support as extensions, such as
flexible array members. So compile with `-pedantic`.
Non-regression for https://github.com/Mbed-TLS/mbedtls/issues/9020.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
