Add libgconf to the runtime.

Adding libgconf to the runtime since games such as Garry's Mod,
Overgrowth and Natural Selection 2 use the Awesomium UI toolkit, which
has a legacy dependency against it. They're working to clean it up, but
haven't been responsive:

http://answers.awesomium.com/questions/1527/why-does-awesomium-on-linux-depend-on-libgconf.html?sort=oldest