xfce: fix typo introduced in recent commit (s/gtk/gtk3/)

Fix typo introduced in commit 0b2dd3a12fca27a185f9c5518acb7d73d140e1e5
(Xfce: Don't depend on GTK+ 3 by default).

(cherry picked from commit b802924af1d8e5d2362e965c437908161e9ec971)