Fix R-package/src/Makevars for OpenCV 4

[leezu]

Description

For OpenCV 4, the pkg-config file is named opencv4.pc instead of opencv.pc.
See for example opencv.pc in Debian Stable https://packages.debian.org/search?searchon=contents&keywords=opencv.pc&mode=exactfilename&suite=stable&arch=any and opencv4.pc in Debian Testing.

Fixes #17359

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage:
• Unit tests are added for small changes to verify correctness (e.g. adding a new operator)
• Nightly tests are added for complicated/long-running ones (e.g. changing distributed kvstore)
• Build tests will be added for build configuration changes (e.g. adding a new build option with NCCL)
• Code is well-documented:
• For user-facing API changes, API doc string has been updated.
• For new C++ functions in header files, their functionalities and arguments are documented.
• For new examples, README.md is added to explain the what the example does, the source of the dataset, expected performance on test set and reference to the original paper if applicable
• Check the API doc at https://mxnet-ci-doc.s3-accelerate.dualstack.amazonaws.com/PR-$PR_ID/$BUILD_ID/index.html
• To the best of my knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change

Changes

• Fix R-package/src/Makevars for OpenCV 4