opencv@3: fix build failure on Linux

[carlocab]

• Have you followed the guidelines for contributing?
• Have you ensured that your commits follow the commit style guide?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
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Follow-up to #131405.

[carlocab]

Didn't work, but should be fixed by Homebrew/brew#15531.