mailbox: zynqmp-ipi-message: 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-rc1/process/deprecated.html#zero-length-and-one-element-arraysSigned-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

include/linux/mailbox/zynqmp-ipi-message.h

@@ -14,7 +14,7 @@
  */
 struct zynqmp_ipi_message {
 	size_t len;
-	u8 data[0];
+	u8 data[];
 };
 
 #endif /* _LINUX_ZYNQMP_IPI_MESSAGE_H_ */