drivers/intel/fsp2_0/fsp_timestamp.c: Use C99 flexible arrays

Use C99 flexible arrays instead of older style of one-element or
zero-length arrays.
It allows the compiler to generate errors when the flexible array does
not occur at the end in the structure.

Change-Id: I03c21e180e9e399e5cb451bf3b9cfb6484cab68b
Signed-off-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/76778
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/drivers/intel/fsp2_0/fsp_timestamp.c

@@ -28,7 +28,7 @@ struct generic_event_record {
 	uint32_t		apic_id;
 	uint64_t		timestamp;
 	uint8_t			guid[16];
-	uint8_t			string[0];
+	uint8_t			string[];
 } __packed;
 
 /*