Set headers as public if needed in PodTargetInstaller

lib/cocoapods/installer/target_installer/pod_target_installer.rb

@@ -55,6 +55,14 @@ module Pod
 
           other_file_refs = other_source_files.map { |sf| project.reference_for_path(sf) }
           native_target.add_file_references(other_file_refs, nil)
+
+          # Set added headers as public if needed
+          if native_target.symbol_type == :framework
+            native_target.headers_build_phase.files.each do |build_file|
+              build_file.settings ||= {}
+              build_file.settings['ATTRIBUTES'] = ['Public']
+            end
+          end
         end
       end