root6: support xrootd

[drbenmorgan]

• [x] Have you followed the guidelines for contributing?
• [x] Have you checked that there aren't other open pull requests for the same formula update/change?
• [x] Have you built your formula locally with brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• [x] Does your build pass brew audit --strict <formula> (after doing brew install <formula>)?

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Fixes #51