avro

Minor: Remove `api` directory from cpp dist (#3042)

* Minor: Remove `api` directory from cpp dist

```
+ ./build.sh dist
-- Version: 10.2.1
-- Build type: Release
Enabled snappy codec
-- Configuring done
-- Generating done
-- Build files have been written to: /home/fokko.driesprong/avro/lang/c++/build
-- Version: 10.2.1
-- Build type: Release
Enabled snappy codec
-- Configuring done
-- Generating done
-- Build files have been written to: /home/fokko.driesprong/avro/lang/c++/build
cp: cannot stat 'api': No such file or directory
``

* Thanks Martin

* Update lang/c++/build.sh

Co-authored-by: Martin Grigorov <martin-g@users.noreply.github.com>

---------

Co-authored-by: Martin Grigorov <martin-g@users.noreply.github.com>