rubocops/resource_requires_dependencies: allow lxml build resource

[cho-m]

Specifically, a lxml build resource can depend on build-only libxml2 and libxslt; however, these were not detected in previous audit since the dependency would be a hash.

Also change message if Linux-only formula.

These changes are both needed for systemd.

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

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This isn't able to handle situation of detecting build-only vs runtime usage of resource. Most likely won't be easily able to do this without extending DSL to annotate this information

However, in the scenario where a build dependency should actually be runtime, our linkage audits should catch it (though won't be a hard failure in case of indirect dependency).

[MikeMcQuaid]

Thanks again @cho-m!

Will let @issyl0 review before merging.

[issyl0]

Thank you!