Missing header from sdist

Building from the sdists result in the Pure python wheel being built since
```
src/google_crc32c/_crc32c.c:3:10: fatal error: crc32c/crc32c.h: No such file or directory
3 | #include
| ^~~~~~~~~~~~~~~~~
compilation terminated.
Compiling the C Extension for the crc32c library failed. Falling back to pure Python build.
running bdist_wheel
```

#### Environment details

- OS type and version: linux
- Python version: -
- pip version: all
- `google-crc32c` version: `1.6.0` - `1.8.0`

#### Steps to reproduce

1.
```bash
pip wheel -v https://files.pythonhosted.org/packages/03/41/4b9c02f99e4c5fb477122cd5437403b552873f014616ac1d19ac8221a58d/google_crc32c-1.8.0.tar.gz
```

2.
```
$ tar --list -f google_crc32c-1.8.0.tar.gz | fgrep -e '.h'
$
```

Thanks!