hw/acpi/pcihp: Clean up global variable shadowing in acpi_pcihp_init()

hw/acpi/pcihp: Clean up global variable shadowing in acpi_pcihp_init()

Fix:

  hw/acpi/pcihp.c:499:36: error: declaration shadows a variable in the global scope [-Werror,-Wshadow]
                       MemoryRegion *address_space_io,
                                     ^
  include/exec/address-spaces.h:35:21: note: previous declaration is here
  extern AddressSpace address_space_io;
                      ^

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Ani Sinha <anisinha@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20231010115048.11856-5-philmd@linaro.org>