opencv3: added tab to make CMake flags aligned

[mdaiter]

Motivation for this change

We should align the OpenCV CMake flags.

Things done

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

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[mention-bot]

@mdaiter, thanks for your PR! By analyzing the history of the files in this pull request, we identified @abbradar, @cpages and @bjornfor to be potential reviewers.

[mdaiter]

cc @viric