meson: fix Cocoa option in summary

Cocoa support was always shown as "no", even it if was enabled.

Fixes: b4e312e9 ("configure: move cocoa option to Meson")
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Chris Hofstaedtler <chris@hofstaedtler.name>
Message-Id: <20201230221623.60423-1-chris@hofstaedtler.name>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>