lib/gcov-io.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: Iad9cbe16a2d1881d74edcc702be843168df8a4ff
Signed-off-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/76846
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>

src/lib/gcov-io.h

@@ -443,7 +443,7 @@ struct gcov_fn_info {
 	gcov_unsigned_t ident;		/* unique ident of function */
 	gcov_unsigned_t lineno_checksum;	/* function lineo_checksum */
 	gcov_unsigned_t cfg_checksum;		/* function cfg checksum */
-	struct gcov_ctr_info ctrs[0];		/* instrumented counters */
+	struct gcov_ctr_info ctrs[];		/* instrumented counters */
 };
 
 /* Type of function used to merge counters.  */