cmake: readd `generate-curl.1` dependency for `src` just in case

cmake: readd `generate-curl.1` dependency for `src` just in case

Trying to fix intermittent build failures with MSVC 2008:
```
unity_0.obj : error LNK2019: unresolved external symbol _showhelp referenced in function _tool_help
unity_0.obj : error LNK2019: unresolved external symbol _hugehelp referenced in function _operate
```
https://ci.appveyor.com/project/curlorg/curl/builds/50700212/job/h3ekljnbccosej5k#L154
https://ci.appveyor.com/project/curlorg/curl/builds/50701615/job/61mkg9og1eooqli8#L155

Follow-up to 496da69aa0eb988ac92eb815742eb89b975ad83e #14883

Closes #15088