ACPI: Cleanup sparse warning on acpi_os_initialize1()

This patch cleans up the following sparse warning:

# make C=2 drivers/acpi/osl.o
...
drivers/acpi/osl.c:1775:20: warning: symbol 'acpi_os_initialize1' was not declared. Should it be static?
...
  CC      drivers/acpi/osl.o

Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/bus.c

@@ -617,7 +617,6 @@ static int __init acpi_bus_init(void)
 {
 	int result;
 	acpi_status status;
-	extern acpi_status acpi_os_initialize1(void);
 
 	acpi_os_initialize1();