hw/i386/acpi_build.c: Avoid shifting left into sign bit

Add U suffix to avoid undefined behaviour.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
