tails::builder: install syslinux-utils from our own repository instead of syslinux from Debian.

Let's use isohybrid 6.03 to hybrid our ISO images. We ship syslinux 6.03 in our
own repository. And in that version, isohybrid is shipped in the
syslinux-utils package.

manifests/builder.pp

@@ -34,7 +34,7 @@ class tails::builder (
     priority => 991,
   }
 
-  apt::preferences_snippet { 'live-build':
+  apt::preferences_snippet { ['live-build', 'syslinux-utils']:
     pin      => 'origin deb.tails.boum.org',
     priority => 991,
   }
@@ -49,7 +49,7 @@ class tails::builder (
     'live-build',
     'perlmagick',
     'po4a',
-    'syslinux',
+    'syslinux-utils',
     'time',
     'whois',
     'intltool',