Enable DEFINES_MODULE for DoubleConversion (#42591)

Summary:
since 32dab7a, `DoubleConversion` is now added as an implicit dependency for 3rd party module and it breaks swift integration. this pr tries to add the `DEFINES_MODULE` to DoubleConversion.

## Changelog:

[IOS] [FIXED] - Fixed `DoubleConversion` build error from Swift integration

Pull Request resolved: #42591

Test Plan:
i'll need to test this on expo latest and react-native nightly build

```sh
# pull latest expo repo and get template tarball
$ git clone --depth 1 https://github.com/expo/expo.git
$ cd expo
$ yarn install
$ cd templates/expo-template-bare-minimum
$ npx pack --pack-destination ../../

# now create an expo app
$ yarn create expo -t blank@sdk-50 sdk50
$ cd sdk50
$ yarn add react-native@nightly
$ jq '.expo.runtimeVersion = { "policy": "appVersion" }' app.json > app.json.tmp && mv app.json.tmp app.json
$ npx expo prebuild -p ios --template /path/to/expo/expo-template-bare-minimum-50.0.17.tgz
$ cd ios
$ pod install
```

then it will show the error message:
```
[!] The following Swift pods cannot yet be integrated as static libraries:

The Swift pod `ExpoModulesCore` depends upon `DoubleConversion`, which does not define modules. To opt into those targets generating module maps (which is necessary to import them from Swift when building as static libraries), you may set `use_modular_headers!` globally in your Podfile, or specify `:modular_headers => true` for particular dependencies.
```

Reviewed By: cortinico

Differential Revision: D53048352

Pulled By: cipolleschi

fbshipit-source-id: b1e27d3d26e8543a4cb2e8062c93c68543a051c5

packages/react-native/third-party-podspecs/DoubleConversion.podspec

@@ -20,6 +20,10 @@ Pod::Spec.new do |spec|
 
   spec.user_target_xcconfig = { "HEADER_SEARCH_PATHS" => "\"$(PODS_ROOT)/DoubleConversion\"" }
 
+  spec.pod_target_xcconfig = {
+    "DEFINES_MODULE" => "YES",
+  }
+
   # Pinning to the same version as React.podspec.
   spec.platforms = min_supported_versions