include/imd_private.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: I53ffa4b35d35d4f8b0170377041b258d4bd2eeeb
Signed-off-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/76777
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/include/imd_private.h

@@ -28,7 +28,7 @@ struct imd_root {
 	uint32_t entry_align;
 	/* Used for fixing the size of an imd. Relative to the root. */
 	int32_t max_offset;
-	struct imd_entry entries[0];
+	struct imd_entry entries[];
 } __packed;
 
 #define IMD_ROOT_PTR_MAGIC  0xc0389481