Build failure on arm-vita

[notgull]

Noticed in this CI job: https://github.com/smol-rs/polling/actions/runs/21573546996/job/62156654034?pr=260

error[E0425]: cannot find function `dirfd` in module `c`
  --> /home/runner/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/rustix-1.1.3/src/backend/libc/fs/dir.rs:91:34
   |
91 |         let raw_fd = unsafe { c::dirfd(self.libc_dir.as_ptr()) };
   |                                  ^^^^^ not found in `c`