Install the cron.d part without the execute bit.

Resolve this lintian error:

	E: zfs-auto-snapshot: bad-permissions-for-etc-cron.d-script etc/cron.d/zfs-auto-snapshot 0755 != 0644
	W: zfs-auto-snapshot: executable-not-elf-or-script etc/cron.d/zfs-auto-snapshot

This should also resolve a related failure on Arch Linux.

Closes: zfsonlinux/zfs-auto-snapshot#4
Thanks: @bitloggerig
Thanks: @scottj97