Install the "crypto" libblockdev plugin (Closes: #14816).

It's needed by recent udisks to do crypto operations,
such as encrypting partitions in GNOME Disks,
or creating a Tails persistent volume.

udisks2 "Recommends: libblockdev-crypto2" but we're (still?) not installing
Recommends by default. So yeah, another case in which we wasted time finding
which package was missing…