Missing parenthesis in call to set.add

[dadrake3]

https://github.com/conda/conda-build/blame/c71c4abee1c85f5a36733c461f224941ab3ebbd1/conda_build/skeletons/cpan.py#L870-L873

This line errors with set.add() takes exactly one argument (2 given), I believe it should be wrapping the args as a tuple. Should be pretty simple fix. Guessing its not covered by unit test.

I can create a PR if thats cool.

[kenodegard]

@dadrake3 Yup missing parenthesis.

And if possible, adding a test would also be swell since you have a use case on hand!

[github-actions]

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