ACPI, x86/boot: make acpi_nobgrt static

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>

arch/x86/kernel/acpi/boot.c

@@ -45,7 +45,7 @@ EXPORT_SYMBOL(acpi_disabled);
 #define PREFIX			"ACPI: "
 
 int acpi_noirq;				/* skip ACPI IRQ initialization */
-int acpi_nobgrt;			/* skip ACPI BGRT */
+static int acpi_nobgrt;			/* skip ACPI BGRT */
 int acpi_pci_disabled;		/* skip ACPI PCI scan and IRQ initialization */
 EXPORT_SYMBOL(acpi_pci_disabled);