Add a zfs-grub dependency to zfs-initramfs.

Add a zfs-grub dependency to zfs-initramfs.

The grub-common package at ppa:dajhorn/zfs-grub provides a
virtual zfs-grub package.

Updating the initrd on a system with a ZFS root filesystem
requires that the grub-probe utility from the grub2 source
package be compiled against a native libzfs.

The /etc/grub.d runparts must also recognize the ZFS root
filesystem so that appropriate kernel command line
parameters are added to the /boot/grub/grub.cfg file.