[Gtk] Make install-dependencies work on Arch Linux
https://bugs.webkit.org/show_bug.cgi?id=136374

Patch by Brendan Long <self@brendanlong.com> on 2014-09-02
Reviewed by Philippe Normand.

* gtk/install-dependencies: Add dependencies for pacman (Arch Linux).

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@173170 268f45cc-cd09-0410-ab3c-d52691b4dbfc