Install libgtk3-simplelist-perl from Jessie backports.

It's required by openpgp-applet.

config/chroot_apt/preferences

@@ -47,6 +47,11 @@ Package: grub2-common
 Pin: origin deb.tails.boum.org
 Pin-Priority: 999
 
+Explanation: required by openpgp-applet
+Package: libgtk3-simplelist-perl
+Pin: release o=Debian,n=jessie-backports
+Pin-Priority: 999
+
 Explanation: src:hplip
 Package: hplip* hpijs-ppds libhpmud* libsane-hpaio printer-driver-hpcups printer-driver-hpijs printer-driver-postscript-hp
 Pin: release o=Debian,n=jessie-backports