ipc/msg: replace one-element array with flexible array member

Replace the deprecated one-element array with a modern flexible array
member in the struct compat_msgbuf.

There are no binary differences after this conversion.

Link: https://github.com/KSPP/linux/issues/79
Link: https://lkml.kernel.org/r/20240930195824.153648-2-thorsten.blum@linux.dev


Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Cc: "Sun, Jiebin" <jiebin.sun@intel.com>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>