Install ekeyd from backports.

The version in Squeeze does not install in chroots, which breaks Tails ISO build
process (Debian bug #624211).

config/chroot_apt/preferences

@@ -10,6 +10,10 @@ Package: eatmydata
 Pin: origin backports.debian.org
 Pin-Priority: 999
 
+Package: ekeyd
+Pin: origin backports.debian.org
+Pin-Priority: 999
+
 Package: florence
 Pin: origin backports.debian.org
 Pin-Priority: 999