[Linux] Fix missed favicons in search engines list

Screenshots (before | after):

![before and after](https://cloud.githubusercontent.com/assets/1687481/4249470/45b29260-3a77-11e4-82a4-5f6eec07fc4e.png)

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1: https://mxr.mozilla.org/mozilla-central/source/browser/components/search/content/search.xml#401

2: https://mxr.mozilla.org/mozilla-central/source/toolkit/content/xul.css#356