Make compatible with -Dwarnings

[dtolnay]

Since rust-lang/rust#121752 in nightly-2024-03-12, rustc reports the following pair of dead_code warnings in build/probe.rs, which makes it look as though error_generic_member_access is not available when using RUSTFLAGS=-Dwarnings (such as in CI).

warning: struct `MyError` is never constructed
  --> build/probe.rs:10:8
   |
10 | struct MyError(Thing);
   |        ^^^^^^^
   |
   = note: `#[warn(dead_code)]` on by default

warning: struct `Thing` is never constructed
  --> build/probe.rs:11:8
   |
11 | struct Thing;
   |        ^^^^^