hw/i386/acpi-build: Avoid 'sun' identifier

On Solaris, 'sun' is #define'd to 1, which causes errors if a variable
is named 'sun'. Slightly change the name of the var for the Slot User
Number so we can build on Solaris.

Reviewed-by: Ani Sinha <ani@anisinha.ca>
Signed-off-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Message-id: 20220316035227.3702-3-adeason@sinenomine.net
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>