Fix cmsg_len() return type on OpenHarmony

[jschwe]

What does this PR do

target_env = "ohos" (OpenHarmony) uses a musl based libc. Fixes a compilation failure when targeting aarch64-unknown-linux-ohos:

error[E0308]: mismatched types
    --> /home/jschwender/.cargo/registry/src/index.crates.io-6f17d22bba15001f/nix-0.29.0/src/sys/socket/mod.rs:1415:32
     |
1415 |             (*cmsg).cmsg_len = self.cmsg_len();
     |             ----------------   ^^^^^^^^^^^^^^^ expected `u32`, found `usize`
     |             |
     |             expected due to the type of this binding

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API

[Chilledheart]

Thanks, Schwender.