Fix an incorrect scoped function call (introduced in #1093)

[mahaloz]

Description

In #1093, a call to add_context_layout_mapping was added without the self scoping, causing a crash when external plugins add a context pane to GEF. I triggered this in the latest version of d2d:

Python Exception <class 'NameError'>: name 'add_context_layout_mapping' is not defined
Error occurred in Python: name 'add_context_layout_mapping' is not defined

Checklist

• My code follows the code style of this project.
• My change includes a change to the documentation, if required.
• If my change adds new code, adequate tests have been added.
• I have read and agree to the CONTRIBUTING document.

Other Recommendations

If possible, I'd recommend adding a linter check as a requirement to the CI, so simple bugs like this can be caught :).

[github-actions]

🤖 Coverage update for 2066ca0 🟢

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