ACPI / platform: include missed header into acpi_platform.c

The internal.h declares the acpi_create_platform_device(). Without
that include we get a following warning:

drivers/acpi/acpi_platform.c:133:24: warning: symbol 'acpi_create_platform_device' was not declared. Should it be static?

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