io_uring: add BUILD_BUG_ON() to assert the layout of struct io_uring_sqe

With nesting of anonymous unions and structs it's hard to
review layout changes. It's better to ask the compiler
for these things.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>