Fix compile with newer astyle

astyle::streamoff was always just an alias for std::streamoff,
see also the upstream commit [1] that removed the
`using namespace std` in the `astyle` namespace which breaks this
code of our here now.

[1]: https://gitlab.com/saalen/astyle/-/commit/0add646b21ee329b231ef7c5ab6963a94fb8935b