systemd: fix syntax error in preset file

Services needs to be named with full unit name.

Fixes QubesOS/qubes-issues#2188

vm-systemd/75-qubes-vm.preset

@@ -30,7 +30,7 @@ disable plymouth-quit-wait.service
 disable smartd.service
 disable upower.service
 disable colord.service
-disable systemd-timesyncd
+disable systemd-timesyncd.service
 
 # Fedora only services
 disable backuppc.service