staging: unisys: visorinput: 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
inadvertenly introduced[3] to the codebase from now on.

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/20200211211722.GA1640@embeddedorSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/unisys/visorinput/visorinput.c

@@ -111,7 +111,7 @@ struct visorinput_devdata {
 	/* size of following array */
 	unsigned int keycode_table_bytes;
 	/* for keyboard devices: visorkbd_keycode[] + visorkbd_ext_keycode[] */
-	unsigned char keycode_table[0];
+	unsigned char keycode_table[];
 };
 
 static const guid_t visor_keyboard_channel_guid = VISOR_KEYBOARD_CHANNEL_GUID;