changelog: Add entry about fixed compilation error in evmc::result::raw()
# Changelog
+## [6.2.2] - unreleased
+
+- Fixed: [[#281](https://github.com/ethereum/evmc/pull/281)]
+ Compilation error of evmc::result::raw() in Visual Studio fixed.
+
## [6.2.1] - 2019-04-29
- Fixed:
