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: KSPP#79
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Thorsten Blum <[email protected]>
Cc: "Sun, Jiebin" <[email protected]>
Cc: Tim Chen <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>