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 <[email protected]>

---------

Co-authored-by: Martin Grigorov <[email protected]>

lang/c++/build.sh

@@ -58,7 +58,7 @@ function do_doc() {
 function do_dist() {
   rm -rf $BUILD_CPP/
   mkdir -p $BUILD_CPP
-  cp -r api AUTHORS build.sh CMakeLists.txt ChangeLog \
+  cp -r include AUTHORS build.sh CMakeLists.txt ChangeLog \
     LICENSE NOTICE impl jsonschemas NEWS README test examples \
     $BUILD_CPP
   find $BUILD_CPP -name '.svn' | xargs rm -rf