Only define the validateFileURL variable in GENERIC builds

There's no point in having this variable defined (implicitly) as `undefined` in e.g. the Firefox PDF Viewer.
By defining it with `var` and using an ESLint ignore, rather than `let`, we can move it into the relevant pre-processor block instead. Note that since the entire viewer-code is placed, by Webpack, in a top-level closure this variable will thus not become globally accessible.

web/app.js

@@ -2178,14 +2178,14 @@ const PDFViewerApplication = {
   },
 };
 
-let validateFileURL;
 if (typeof PDFJSDev === "undefined" || PDFJSDev.test("GENERIC")) {
   const HOSTED_VIEWER_ORIGINS = [
     "null",
     "http://mozilla.github.io",
     "https://mozilla.github.io",
   ];
-  validateFileURL = function (file) {
+  // eslint-disable-next-line no-var
+  var validateFileURL = function (file) {
     if (!file) {
       return;
     }