fix: codegen generated headers are public on ios (#737)

### Summary

Fixes #690

### Test plan

Author: atlj

packages/create-react-native-library/templates/native-common/{%- project.identifier %}.podspec

@@ -20,6 +20,7 @@ Pod::Spec.new do |s|
   s.source_files = "ios/**/*.{h,m,mm,swift}"
 <% } else if (project.arch !== "legacy") { -%>
   s.source_files = "ios/**/*.{h,m,mm,cpp}"
+  s.private_header_files = "ios/generated/**/*.h"
 <% } else { -%>
   s.source_files = "ios/**/*.{h,m,mm}"
 <% } -%>