Unused variable warning when building with g++-7

[sbc100]

We build binaryen with g++-7 as part of emsdk CI and we are getting unused variable warning regarding:

https://github.com/WebAssembly/binaryen/blob/3c1c8ae829d0026024de55aa65dc3d49bd8fd37a/src/passes/GlobalRefining.cpp#L57-L61

See https://app.circleci.com/pipelines/github/emscripten-core/emsdk/1385/workflows/d696706d-f612-429d-94ae-12b0742e7cfb/jobs/7067?invite=true#step-105-303

This seems to not effect g++-8 and above or clang++ or mscv I guess (otherwise binaryen CI would have failed).

Its not clear we want to continue to support g++-7 but filing this issue so I can reference it in my emsdk change.

Related PR: https://github.com/WebAssembly/binaryen/pull/3104