lib/cbmem_console.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: I3d716b29d8e28584a0c9e4056d4c93dca2873114
Signed-off-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/76780
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/lib/cbmem_console.c

@@ -28,7 +28,7 @@
 struct cbmem_console {
 	u32 size;
 	u32 cursor;
-	u8  body[0];
+	u8  body[];
 }  __packed;
 
 #define MAX_SIZE (1 << 28)	/* can't be changed without breaking readers! */