Suppress warnings from cryptsetup initramfs hook.

This should fix the "unstable" diffoscope builds, eg:

  https://jenkins.debian.net/job/reproducible_diffoscope_from_git_master/555/parsed_console/

For example:

  Setting up linux-image-4.18.0-2-amd64 (4.18.10-2) ...
  I: /vmlinuz.old is now a symlink to boot/vmlinuz-4.18.0-2-amd64
  I: /initrd.img.old is now a symlink to boot/initrd.img-4.18.0-2-amd64
  I: /vmlinuz is now a symlink to boot/vmlinuz-4.18.0-2-amd64
  I: /initrd.img is now a symlink to boot/initrd.img-4.18.0-2-amd64
  /etc/kernel/postinst.d/initramfs-tools:
  update-initramfs: Generating /boot/initrd.img-4.18.0-2-amd64
  cryptsetup: WARNING: Couldn't determine root device
  cryptsetup: ERROR: Couldn't resolve device /dev/dm-2
  cryptsetup: WARNING: The initramfs image may not contain cryptsetup binaries
      nor crypto modules. If that's on purpose, you may want to uninstall the
      'cryptsetup-initramfs' package in order to disable the cryptsetup initramfs
      integration and avoid this warning.
  loadkeys is missing. Please install the 'kbd' package.
  Setting up policykit-1 (0.105-21) ...
  Setting up cryptsetup (2:2.0.4-3) ...