fix(additional-assets): adds missing method

When using the plugin with `extract: true` config option along `compression-webpack-plugin`, `webpack.cache.PackFileCacheStrategy` crashes.

This fixes the issue by defining the expected method.

lib/plugin.js

@@ -168,7 +168,8 @@ class SVGSpritePlugin {
 
           compilation.assets[`${this.filenamePrefix}${filename}`] = {
             source() { return content; },
-            size() { return content.length; }
+            size() { return content.length; },
+            updateHash(bulkUpdateDecorator) { bulkUpdateDecorator.update(content); }
           };
         });
     });