iconGrid: Fix nonsense condition

Just like the "in" operator in the previous patch, "instanceof" has
a lower precedence than negation, resulting in the nonsense condition
of "true instanceof BaseIcon".

Add parentheses to get the intended behavior.

Spotted by eslint.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/375

js/ui/iconGrid.js

@@ -670,7 +670,7 @@ var IconGrid = GObject.registerClass({
     }
 
     addItem(item, index) {
-        if (!item.icon instanceof BaseIcon)
+        if (!(item.icon instanceof BaseIcon))
             throw new Error('Only items with a BaseIcon icon property can be added to IconGrid');
 
         this._items.push(item);