Bug#897568: zfs-dkms 0.7.6 fails to build on 4.16.0-1-amd64

[John Allen]

According to the release notes (
https://github.com/zfsonlinux/zfs/releases/tag/zfs-0.7.8), the latest
version (0.7.8) is needed to build on the 4.16 kernel.

The latest version in Debian is still 0.7.6 (in testing since 7 March
2018).

John