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>

drivers/acpi/internal.h

@@ -11,6 +11,8 @@
 
 #include <linux/idr.h>
 
+extern struct acpi_device *acpi_root;
+
 int early_acpi_osi_init(void);
 int acpi_osi_init(void);
 acpi_status acpi_os_initialize1(void);

drivers/acpi/scan.c

@@ -24,8 +24,6 @@
 
 #include "internal.h"
 
-extern struct acpi_device *acpi_root;
-
 #define ACPI_BUS_CLASS			"system_bus"
 #define ACPI_BUS_HID			"LNXSYBUS"
 #define ACPI_BUS_DEVICE_NAME		"System Bus"