printk: ringbuffer: Replace zero-length array with flexible-array member

There is a regular need in the kernel to provide a way to declare having a
dynamically sized set of trailing elements in a structure. Kernel code should
always use “flexible array members”[1] for these cases. The older style of
one-element or zero-length arrays should no longer be used[2].

[1] https://en.wikipedia.org/wiki/Flexible_array_member
[2] https://www.kernel.org/doc/html/v5.9/process/deprecated.html#zero-length-and-one-element-arraysSigned-off-by: NGustavo A. R. Silva <gustavoars@kernel.org>

kernel/printk/printk_ringbuffer.c

@@ -345,7 +345,7 @@ DESC_ID((id) - DESCS_COUNT(desc_ring))
  */
 struct prb_data_block {
 	unsigned long	id;
-	char		data[0];
+	char		data[];
 };
 
 /*