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

Yoga.podspec

@@ -30,6 +30,7 @@ Pod::Spec.new do |spec|
       '-fexceptions',
       '-Wall',
       '-Werror',
+      '-Wextra',
       '-std=c++14',
       '-fPIC'
   ]