See commit de6cd759 ("meson: Replace softmmu_ss -> system_ss")
for rationale.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20231004090629.37473-12-philmd@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>