The configure option was backward, and we failed to
pass the value on to meson.

Fixes: 23a77b2d ("build-system: clean up TCG/TCI configury")
Tested-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>