Fix the following sparse warning:

arch/x86/kernel/acpi/boot.c:48:5: warning: symbol 'acpi_nobgrt' was not
declared. Should it be static?
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>