tools headers: Grab copy of io_uring.h

This file will be used by mini_uring.h and allow tests to run without
the need of installing liburing to run the tests.

This is needed to run io_uring tests in BPF, such as
(tools/testing/selftests/bpf/prog_tests/sockopt.c).

Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://lore.kernel.org/r/20231016134750.1381153-7-leitao@debian.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>