Need to copy loading.html

Summary: Need to copy this resource too from the static directory.

Reviewed By: aigoncharov

Differential Revision: D48864174

fbshipit-source-id: 0f3bfe9608bb9d48ec1cfb7243b3f0701d2fd8cf

desktop/scripts/build-flipper-server-release.tsx

@@ -234,6 +234,7 @@ async function copyStaticResources(outDir: string, versionNumber: string) {
     'icons.json',
     'index.web.dev.html',
     'index.web.html',
+    'loading.html',
     'offline.html',
     'service-worker.js',
     'style.css',