[GTK] install-dependencies don't installs Xvfb that is needed for the tests.
https://bugs.webkit.org/show_bug.cgi?id=136153

Reviewed by Martin Robinson.

* gtk/install-dependencies: Add Xvfb package to the list of dependencies necessary for running tests.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@172858 268f45cc-cd09-0410-ab3c-d52691b4dbfc