m68k: amiga: config: Replace zero-length array with flexible-array member

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732 ("cxgb3/l2t: Fix undefined behaviour")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Link: https://lore.kernel.org/r/20200420181401.GA32172@embeddedorSigned-off-by: Geert Uytterhoeven <geert@linux-m68k.org>

arch/m68k/amiga/config.c

@@ -629,7 +629,7 @@ struct savekmsg {
 	unsigned long magic2;		/* SAVEKMSG_MAGIC2 */
 	unsigned long magicptr;		/* address of magic1 */
 	unsigned long size;
-	char data[0];
+	char data[];
 };
 
 static struct savekmsg *savekmsg;