[GTK] update-webkitgtk-libs needs itstool

[GTK] update-webkitgtk-libs needs itstool
https://bugs.webkit.org/show_bug.cgi?id=130082

After http://trac.webkit.org/changeset/165110 WebKitGTK+ uses gtk-doc 1.20.
itstool is a dependency of building gtk-doc from version 1.19, so it should
be added to Tools/gtk/install-dependencies script, too.

Patch by Martin Hodovan <mhodovan@inf.u-szeged.hu> on 2014-03-11
Reviewed by Philippe Normand.

* gtk/install-dependencies:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@165448 268f45cc-cd09-0410-ab3c-d52691b4dbfc