[node] Work around a link error on macOS release builds

[jfirebaugh]

Fixes:

module.js:434
  return process.dlopen(module, path._makeLong(filename));
                 ^

Error: dlopen(/Users/john/Development/mapbox-gl-native/lib/mapbox_gl_native.node, 1): Symbol not found: __ZNK4mbgl5style5Layer2isINS0_11CustomLayerEEEbv
  Referenced from: /Users/john/Development/mapbox-gl-native/lib/mapbox_gl_native.node
  Expected in: flat namespace
 in /Users/john/Development/mapbox-gl-native/lib/mapbox_gl_native.node
    at Error (native)
    at Object.Module._extensions..node (module.js:434:18)
    at Module.load (module.js:343:32)
    at Function.Module._load (module.js:300:12)
    at Module.require (module.js:353:17)
    at require (internal/module.js:12:17)
    at Object.<anonymous> (/Users/john/Development/mapbox-gl-native/platform/node/index.js:5:12)
    at Module._compile (module.js:409:26)
    at Object.Module._extensions..js (module.js:416:10)
    at Module.load (module.js:343:32)

I don't know why this occurs.

cc @bsudekum

[mention-bot]

@jfirebaugh, thanks for your PR! By analyzing this pull request, we identified @1ec5 and @ansis to be potential reviewers.

[bsudekum]

👍