Ignore needless_update clippy lint

With #[cfg...], sometimes all the fields containing updates are
configured out and that's fine.

    warning: struct update has no effect, all the fields in the struct have already been specified
       --> src/fixup.rs:235:15
        |
    235 |             ..self.leftmost_subexpression()
        |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_update
        = note: `#[warn(clippy::needless_update)]` on by default

src/lib.rs

@@ -285,6 +285,7 @@
     clippy::must_use_candidate,
     clippy::needless_doctest_main,
     clippy::needless_pass_by_value,
+    clippy::needless_update,
     clippy::never_loop,
     clippy::range_plus_one,
     clippy::redundant_else,