multimedia/rav1e: update to 0.2.1

Changes: https://github.com/xiph/rav1e/releases/tag/v0.2.1
Changes: https://github.com/xiph/rav1e/compare/v0.2.0...v0.2.1
MFH: 2020Q1 (no ABI changes, only 1 non-default consumer, stability fixes)
Reported by: GitHub (watch releases)