[elixir] add files to package (#257)

Author: schmit

.changeset/tidy-trees-rhyme.md

@@ -0,0 +1,5 @@
+---
+"elixir-sdk": patch
+---
+
+Add native/core_sdk to package.files

elixir-sdk/mix.exs

@@ -49,7 +49,8 @@ defmodule EppoSdk.MixProject do
         "GitHub" => "https://github.com/Eppo-exp/eppo-multiplatform/tree/main/elixir-sdk",
         "Eppo" => "https://www.geteppo.com",
         "Documentation" => "https://docs.geteppo.com/sdks/server-sdks/"
-      }
+      },
+      files: ~w(lib native/sdk_core mix.exs README.md LICENSE)
     ]
   end
 end