Fix build on Haiku OS

[jedenastka]

This disables async-process on Haiku OS, so smol as a whole is able to build. The same solution seems to be already in place for the espidf target, so I guess this should be okay.

[taiki-e]

Thanks for the PR, but the correct way to fix the issue is to fix async-signal. Filed smol-rs/async-signal#59.

Looking at the error messages, it should be clear that those from Haiku and ESP-IDF are of entirely different natures.

On Haiku (just lacking SIGIO):

error[E0599]: no associated item named `IO` found for struct `rustix::process::Signal` in the current scope
   --> /Users/taiki/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/async-signal-0.2.13/src/lib.rs:137:35
    |
 99 |                 rustix::process::Signal::$rustix_name.as_raw()
    |                                          ------------ due to this macro variable
...
137 |     pub const SIGIO: c_int = sig!(IO, 29);
    |                                   ^^ associated item not found in `rustix::process::Signal`

On ESP-IDF (lacking entirely sig*):

error[E0432]: unresolved import `libc::siginfo_t`
  --> /Users/taiki/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/signal-hook-registry-1.4.6/src/lib.rs:82:38
   |
82 | use libc::{c_int, c_void, sigaction, siginfo_t};
   |                                      ^^^^^^^^^ no `siginfo_t` in the root

error[E0432]: unresolved imports `libc::SIGKILL`, `libc::SIGSTOP`
  --> /Users/taiki/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/signal-hook-registry-1.4.6/src/lib.rs:87:28
   |
87 | use libc::{SIGFPE, SIGILL, SIGKILL, SIGSEGV, SIGSTOP};
   |                            ^^^^^^^           ^^^^^^^ no `SIGSTOP` in the root
   |                            |
   |                            no `SIGKILL` in the root
   |                            help: a similar name exists in the module: `SIGILL`

error[E0425]: cannot find value `SA_RESTART` in crate `libc`
   --> /Users/taiki/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/signal-hook-registry-1.4.6/src/lib.rs:189:27
    |
189 |         let flags = libc::SA_RESTART;
    |                           ^^^^^^^^^^ not found in `libc`

error[E0425]: cannot find value `SA_SIGINFO` in crate `libc`
   --> /Users/taiki/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/signal-hook-registry-1.4.6/src/lib.rs:192:25
    |
192 |         siginfo = libc::SA_SIGINFO as _;
    |                         ^^^^^^^^^^ not found in `libc`

error[E0425]: cannot find value `SA_SIGINFO` in crate `libc`
   --> /Users/taiki/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/signal-hook-registry-1.4.6/src/lib.rs:270:29
    |
270 |             siginfo = libc::SA_SIGINFO as _;
    |                             ^^^^^^^^^^ not found in `libc`

error[E0609]: no field `sa_sigaction` on type `sigaction`
   --> /Users/taiki/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/signal-hook-registry-1.4.6/src/lib.rs:180:13
    |
180 |         new.sa_sigaction = handler as usize; // If it doesn't compile on AIX, upgrade the libc dependency
    |             ^^^^^^^^^^^^ unknown field
    |
    = note: available fields are: `sa_handler`, `sa_mask`, `sa_flags`

error[E0609]: no field `sa_sigaction` on type `sigaction`
   --> /Users/taiki/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/signal-hook-registry-1.4.6/src/lib.rs:259:30
    |
259 |         let fptr = self.info.sa_sigaction;
    |                              ^^^^^^^^^^^^ unknown field
    |
    = note: available fields are: `sa_handler`, `sa_mask`, `sa_flags`