system: Enable acpi_call kernel module.

system/config.scm

@@ -46,6 +46,9 @@
 (operating-system
  ;; Use LTS versions of the kernel.
  (kernel linux-libre-lts)
+ (kernel-loadable-modules
+  (list
+   acpi-call-linux-module))
  (kernel-arguments
   (append
    (list
@@ -176,6 +179,9 @@
               (device-type "wacom")
               (device "/dev/ttyS4")
               (baud-rate 38400)))
+    (service kernel-module-loader-service-type
+             (list
+              "acpi_call"))
     (service sane-service-type)
     (service udisks-service-type
              (udisks-configuration))