Fix hex package to include priv directory

Author: benoitc

rebar.config

@@ -28,7 +28,10 @@
 
 {project_plugins, [rebar3_ex_doc]}.
 
-{hex, [{doc, ex_doc}]}.
+{hex, [
+    {doc, ex_doc},
+    {files, ["src", "include", "priv", "c_src", "CMakeLists.txt", "do_cmake.sh", "do_build.sh", "rebar.config", "rebar.lock", "README.md", "LICENSE"]}
+]}.
 
 {ex_doc, [
     {source_url, <<"https://github.com/benoitc/erlang-python">>},