fix: provide correct serialization context

[vankop]

instead of context use local variable ctx

What kind of change does this PR introduce?

bugfix

Did you add tests for your changes?

no

Does this PR introduce a breaking change?

no

What needs to be documented once your changes are merged?

nothing

[webpack-bot]

For maintainers only:

• This needs to be documented (issue in webpack/webpack.js.org will be filed when merged)
• This needs to be backported to webpack 4 (issue will be created when merged)

[webpack-bot]

Thank you for your pull request! The most important CI builds succeeded, we’ll review the pull request soon.

[sokra]

Thanks