Add missing libgtk as a direct dependency, no longer installed indirectly

Change-Id: If8554809f44984a8bf6695bc1799b12f0e7900ab
diff --git a/tasks/Debian.yml b/tasks/Debian.yml
index 6483831..7e2c5ab 100644
--- a/tasks/Debian.yml
+++ b/tasks/Debian.yml
@@ -11,6 +11,7 @@
       - "git"
       - "ghostscript"
       - "graphicsmagick"
+      - "libgtk-3-0"
     state: "present"
     update_cache: true
     cache_valid_time: 3600