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: 29bf71006f63161521fe5869c3a7d8bf7aae9c81

tests/util/TestUtil.cpp

@@ -19,9 +19,9 @@ int nodeInstanceCount = 0;
 namespace {
 
 void yogaEventSubscriber(
-    const YGNode& node,
+    const YGNode& /*node*/,
     Event::Type eventType,
-    const Event::Data& eventData) {
+    const Event::Data& /*eventData*/) {
 
   switch (eventType) {
     case Event::NodeAllocation: