ACPI: APEI build fix

as GHES is optional...

When # CONFIG_ACPI_APEI_GHES is not set:

(.init.text+0x4c22): undefined reference to `ghes_disable'

Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Len Brown <len.brown@intel.com>
diff --git a/include/acpi/apei.h b/include/acpi/apei.h
index d40bc55..51a527d 100644
--- a/include/acpi/apei.h
+++ b/include/acpi/apei.h
@@ -18,7 +18,11 @@
 
 extern int hest_disable;
 extern int erst_disable;
+#ifdef CONFIG_ACPI_APEI_GHES
 extern int ghes_disable;
+#else
+#define ghes_disable 1
+#endif
 
 #ifdef CONFIG_ACPI_APEI
 void __init acpi_hest_init(void);