Fix build on architectures where char is unsigned

[cho-m]

When building on aarch64 (armv8-a) Linux (GitHub's ubuntu-22.04-arm runner and local VM), there was a build failure:

   Compiling ipc v0.0.1 (https://github.com/octobuild/ipc-rs#e8d76ee3)
error[E0308]: mismatched types
   --> /home/linuxbrew/.cache/Homebrew/cargo_cache/git/checkouts/ipc-rs-a7ac1f2a1bbcf2f1/e8d76ee/src/unix.rs:156:13
    |
155 |         let fd = libc::open(
    |                  ---------- arguments to this function are incorrect
156 |             filename.as_ptr() as *const i8,
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `*const u8`, found `*const i8`
    |
    = note: expected raw pointer `*const u8`
               found raw pointer `*const i8`

This seems due to how some platforms can define char as unsigned.

libc seems to take this into account via the libc::c_char:

• https://docs.rs/libc/latest/src/libc/primitives.rs.html#46