Cast the 'cap' argument of 'getgrnam_r' to 'i32' on AIX

[obeton]

What does this PR do

The AIX signature of getgrnam_r(...) differs from POSIX specification, which expects size_t (rust: usize) as the type of the cap/buflen argument, whereas AIX uses int (rust: i32). This patch casts cap inside the nix wrapper to avoid the mismatch

Example of compilation failure from cargo build output:

error[E0308]: mismatched types --> /home/XXXX/.cargo/registry/src/index.crates.io-7f555b6b8ccf4919/nix-0.28.0/src/unistd.rs:3587:60
3587 | libc::getgrnam_r(name.as_ptr(), grp, cbuf, cap, res)
| ---------------- ^^^ expected i32, found usize
| arguments to this function are incorrect
|
note: function defined here
--> /home/XXX/.cargo/registry/src/index.crates.io-7f555b6b8ccf4919/libc-0.2.175/src/unix/aix/mod.rs:2926:12

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