efi: add NVRAM variable GUID definition

Change-Id: Iab4e59bb4ac91ba5e9c34e1ade433b423a140b9b
Reviewed-on: https://review.haiku-os.org/c/haiku/+/4316
Reviewed-by: Fredrik Holmqvist <fredrik.holmqvist@gmail.com>

headers/private/kernel/platform/efi/runtime-services.h

@@ -32,6 +32,9 @@
 #define EFI_HARDWARE_ERROR_VARIABLE \
     {0x414e6bdd, 0xe47b, 0x47cc, {0xb2, 0x44, 0xbb, 0x61, 0x02, 0x0c, 0xf5, 0x16}}
 
+#define EFI_GLOBAL_VARIABLE \
+	{0x8BE4DF61, 0x93CA, 0x11d2, {0xAA, 0x0D, 0x00, 0xE0, 0x98, 0x03, 0x2B, 0x8C}}
+
 typedef struct {
     uint16_t Year;
     uint8_t  Month;