Input: applespi - replace zero-length array with DECLARE_FLEX_ARRAY() helper

Zero-length arrays are deprecated and we are moving towards adopting
C99 flexible-array members, instead. So, replace zero-length arrays
declarations in anonymous union with the new DECLARE_FLEX_ARRAY()
helper macro.

This helper allows for flexible-array members in unions.

Link: https://github.com/KSPP/linux/issues/193
Link: https://github.com/KSPP/linux/issues/219
Link: https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/YzIeJeqU73G+UI8g@work
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/keyboard/applespi.c

@@ -311,7 +311,7 @@ struct message {
 		struct command_protocol_mt_init	init_mt_command;
 		struct command_protocol_capsl	capsl_command;
 		struct command_protocol_bl	bl_command;
-		u8				data[0];
+		DECLARE_FLEX_ARRAY(u8, 		data);
 	};
 };