Add modular headers support to Objective-C Firebase pods for static framework compatibility (#15506)

Co-authored-by: Nick Cooke <36927374+ncooke3@users.noreply.github.com>

FirebaseCoreExtension.podspec

@@ -34,5 +34,9 @@ Pod::Spec.new do |s|
       "#{s.module_name}_Privacy" => 'FirebaseCore/Extension/Resources/PrivacyInfo.xcprivacy'
     }
 
+    s.pod_target_xcconfig = {
+      'DEFINES_MODULE' => 'YES'
+    }
+
     s.dependency 'FirebaseCore', '~> 12.7.0'
   end