ACPI: suppress compiler warning in container.c

This patch fixes following compiler warning when build via make W=1:

drivers/acpi/container.c:183:116: warning: no previous prototype for ‘acpi_container_init’ [-Wmissing-prototypes]

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