Fix warning/error with gcc11 in PalRedhawkUnix.cpp (#82596)

Fix warning/error with gcc11 in PalRedhawkUnix.cpp (#82596)

Otherwise you get:

```
error: ignoring return value of ‘ssize_t write(int, const void*, std::size_t)’ declared with attribute ‘warn_unused_result’ [-Werror=unused-result]
    681 |     write(STDERR_FILENO, message, strlen(message));
        |     ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

The same warning was already fixed for `alloca()` in encee.cpp as part of https://github.com/dotnet/runtime/commit/5d6d55977270b3d1549a5c2e454b191487bef5eb#diff-f1018f796e7e06f60f7f4558583ec116f8e394bc90e29177c9dbdbfd29b78ab5R670, applying the same fix here