gdal: Add zstandard compression support

[rcoup]

Adds zstandard compression support via a dependency on the existing zstd brew formula.

Background: http://osgeo-org.1560.x6.nabble.com/gdal-dev-ZSTD-compression-for-GeoTIFF-td5354009.html

---

• Have you followed the guidelines for contributing?
• 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?
• Does your build pass brew audit --strict <formula> (after doing brew install <formula>)?

[fxcoudert]

Thanks @rcoup for the pull request!