wasm/js: fix invalid restartCount reference

The `restartCount` variable on line 245 is mutating
global scope. The PR makes it consistent with the
rest ob the code (`self.restartCount`).

It was observed when importing the qtloader in a
typical Webpack/Babel build environment.

Change-Id: I338285f4f6bcb80df0c16d80cc3ebfec944a8be7
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

src/plugins/platforms/wasm/qtloader.js

@@ -242,7 +242,7 @@ function QtLoader(config)
     publicAPI.setFontDpi = setFontDpi;
     publicAPI.fontDpi = fontDpi;
 
-    restartCount = 0;
+    self.restartCount = 0;
 
     function fetchResource(filePath) {
         var fullPath = config.path + filePath;