ACPI: scan: Move acpi_root to internal header

Compiler is not happy about handling of acpi_root variable:

  ...drivers/acpi/bus.c:37:20: warning: symbol 'acpi_root' was not declared. Should it be static?

Move it's definition to the internal header.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>