sched: Correctly sort struct predeclarations

The comment says:
  /* task_struct member predeclarations (sorted alphabetically): */

So move io_uring_task where it belongs (alphabetically).

Signed-off-by: Peter Oskolkov <posk@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210126193449.487547-1-posk@google.com