commonlib/timestamp_serialized.h: 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: Ibd1e4bc96a2f5eea746328a09d123629c20b272c
Signed-off-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/76847
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>

src/commonlib/include/commonlib/timestamp_serialized.h

@@ -16,7 +16,7 @@ struct timestamp_table {
 	uint16_t	max_entries;
 	uint16_t	tick_freq_mhz;
 	uint32_t	num_entries;
-	struct timestamp_entry entries[0]; /* Variable number of entries */
+	struct timestamp_entry entries[]; /* Variable number of entries */
 } __packed;
 
 enum timestamp_id {