io_uring: dual license io_uring.h uapi header

This just syncs the header it with the liburing version, so there's no
confusion on the license of the header parts.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/uapi/linux/io_uring.h

@@ -1,4 +1,4 @@
-/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note OR MIT */
 /*
  * Header file for the io_uring interface.
  *