meson: add back -Werror=vla to msvc compat flags

meson: add back -Werror=vla to msvc compat flags

Back when STATIC_ASSERT was prepared for use in common code, the
-Werror=vla flag was removed from the MSVC compat flags. But now we're
using C++11 / C11 static asserts instead, so we can add it back again.

This should help us noticing some breakages before they happen.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16670>