[Bug-Candidate]: Flat don't include any license and don't compile

### Describe the issue:

Now seems mandatory the SPDX license, and is not included

```
ERROR:CryticCompile:Invalid solc compilation Warning: SPDX license identifier not provided in source file. Before publishing, consider adding a comment containing "SPDX-License-Identifier: " to each source file. Use "SPDX-License-Identifier: UNLICENSED" for non-open-source code. Please see https://spdx.org for more information.
--> /sharedVolume/0197f034-62e4-339f-2416-775d76ac8679/export.sol
```

### Code example to reproduce the issue:

slither-flat path --strategy OneFile --convert-external --dir outPath

### Version:

FROM ghcr.io/trailofbits/eth-security-toolbox/ci@sha256:dddfd40e9f055b6f092b0711d5827ebd0d9beb30aaadbd8c8d51d89bdba37442