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: #4
Thanks: @bitloggerig
Thanks: @scottj97

Makefile

@@ -8,7 +8,7 @@ install:
 	install -d $(DESTDIR)/etc/cron.hourly
 	install -d $(DESTDIR)/etc/cron.weekly
 	install -d $(DESTDIR)/etc/cron.monthly
-	install etc/zfs-auto-snapshot.cron.frequent $(DESTDIR)/etc/cron.d/zfs-auto-snapshot
+	install -m 0644 etc/zfs-auto-snapshot.cron.frequent $(DESTDIR)/etc/cron.d/zfs-auto-snapshot
 	install etc/zfs-auto-snapshot.cron.hourly   $(DESTDIR)/etc/cron.hourly/zfs-auto-snapshot
 	install etc/zfs-auto-snapshot.cron.daily    $(DESTDIR)/etc/cron.daily/zfs-auto-snapshot
 	install etc/zfs-auto-snapshot.cron.weekly   $(DESTDIR)/etc/cron.weekly/zfs-auto-snapshot