services: fstrim: Fix schedule ungexp.

Previously, only the first level of the list would be quoted,
resulting in a schedule of the sort:
'(next-second (range 0 60 30))
being incorrectly ungexp'd into:
(list next-second (0 30))

* gnu/services/linux.scm (fstrim-mcron-job): Fix schedule ungexp.

Signed-off-by: Maxim Cournoyer <maxim.cournoyer@gmail.com>

gnu/services/linux.scm

@@ -234,7 +234,7 @@ (define (fstrim-mcron-job config)
        ;; lists are ungexp'd correctly since @var{schedule}
        ;; can be either a procedure, a string or a list.
        #$(if (list? schedule)
-             `(list ,@schedule)
+             #~'(#$@schedule)
              schedule)
        (lambda ()
          (system* #$(file-append package "/sbin/fstrim")