Improve Preferences list

Author: athos-ribeiro

salt/files/debian_unstable.conf

@@ -60,6 +60,8 @@ Keep: gzip libada libstdc++ libunwind
 Keep: libunwind-devel libzio make mktemp pam-devel pam-modules
 Keep: patch perl rcs timezone
 
+Prefer: libjpeg-dev
+
 Prefer: cvs libesd0 libfam0 libfam-dev expect
 
 Prefer: gawk locales default-jdk