awesome: Allows awesome to load svg images.

Awesome can load SVG images, just like it can do PNG, through gdk.

The support for SVG images through GDK needs librvsg.

This commits adds the plumbing necessary in the wrapper that makes
awesome be able to load SVG images.

Without this, awesome will not load SVG images. The related error message is:

 > Couldn't recognize the image file format for file

Which comes from gdk.