src: squelch -Wunused-variable in non-icu builds

Silence the following compiler warning when building without ICU:

    ../src/node_config.cc:32:16: warning: unused variable 'env'
    [-Wunused-variable]
       Environment* env = Environment::GetCurrent(context);

PR-URL: https://github.com/nodejs/node/pull/6351


Reviewed-By: James M Snell <jasnell@gmail.com>