selftests: ublk: add io_uring uapi header

Add io_uring UAPI header so that ublk can work with latest uapi
definition.

Fix the following build failure:

stripe.c: In function ‘stripe_to_uring_op’:
stripe.c:120:29: error: ‘IORING_OP_READV_FIXED’ undeclared (first use in this function); did you mean ‘IORING_OP_READ_FIXED’?
  120 |                 return zc ? IORING_OP_READV_FIXED : IORING_OP_READV;
      |                             ^~~~~~~~~~~~~~~~~~~~~
      |                             IORING_OP_READ_FIXED

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Fixes: 57ed58c ("selftests: ublk: enable zero copy for stripe target")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250412023035.2649275-3-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Ming Lei

tools/testing/selftests/ublk/kublk.h

@@ -20,6 +20,7 @@
 #include <sys/wait.h>
 #include <sys/eventfd.h>
 #include <sys/uio.h>
+#include <linux/io_uring.h>
 #include <liburing.h>
 #include <linux/ublk_cmd.h>
 #include "ublk_dep.h"