tools headers UAPI: Sync openat2.h with the kernel sources

To pick the changes in:

  99668f61 ("fs: expose LOOKUP_CACHED through openat2() RESOLVE_CACHED")

That don't result in any change in tooling, only silences this perf
build warning:

  Warning: Kernel ABI header at 'tools/include/uapi/linux/openat2.h' differs from latest version at 'include/uapi/linux/openat2.h'
  diff -u tools/include/uapi/linux/openat2.h include/uapi/linux/openat2.h

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>