Enable -Wextra in C++ builds (#1294)

Summary:
Pull Request resolved: https://github.com/facebook/yoga/pull/1294

X-link: https://github.com/facebook/react-native/pull/37383

Add -Wextra to the build, and fixup some more instances of -Wunused-parameter that it sufaces which were not automatically fixable.

Reviewed By: javache

Differential Revision: D45772846

fbshipit-source-id: 680498f66f2104c29a71366bf30dba8895d9baf4

tests/EventsTest.cpp

@@ -290,7 +290,7 @@ EventArgs createArgs(const YGNode& node, const Event::Data data) {
   using Data = Event::TypedData<E>;
   auto deleteData = [](void* x) { delete static_cast<Data*>(x); };
 
-  return {&node, E, {new Data{(data.get<E>())}, deleteData}};
+  return {&node, E, {new Data{(data.get<E>())}, deleteData}, nullptr};
 }
 
 template <Event::Type E>