soc/intel/broadwell/pch/me.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: Iea63e7ce165b1c8129725136e39bff45765023e6
Signed-off-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/76782
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

src/soc/intel/broadwell/pch/me.c

@@ -806,7 +806,7 @@ static u32 me_to_host_words_pending(void)
 
 struct mbp_payload {
 	mbp_header header;
-	u32 data[0];
+	u32 data[];
 };
 
 /*