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>

tools/include/uapi/linux/openat2.h

@@ -35,5 +35,9 @@ struct open_how {
 #define RESOLVE_IN_ROOT		0x10 /* Make all jumps to "/" and ".."
 					be scoped inside the dirfd
 					(similar to chroot(2)). */
+#define RESOLVE_CACHED		0x20 /* Only complete if resolution can be
+					completed through cached lookup. May
+					return -EAGAIN if that's not
+					possible. */
 
 #endif /* _UAPI_LINUX_OPENAT2_H */