meson_options.txt: Enable qom-cast-debug by default again

This switch had been disabled by default by accident in commit
c55cf6ab. But we should enable it by default instead to avoid
regressions in the QOM device hierarchy.

Fixes: c55cf6ab ("configure, meson: move some default-disabled options to meson_options.txt")
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20230417130037.236747-3-thuth@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>