systemd-preset: enable fstrim.timer

On LVM thin it is easy to fill the pool if fstrim (or 'discard' mount
option) isn't used from time to time. Enable fstrim.timer by default,
which will do fstrim once a week.

system-config/75-qubes-dom0.preset

@@ -16,6 +16,7 @@ enable lvm2-monitor.*
 enable lvm2-lvmetad.*
 enable dm-event.*
 enable dmraid-activation.service
+enable fstrim.timer
 
 
 enable abrtd.service