services: Add libmtp's udev rules to '%desktop-services'.

Suggested by Chris Marusich <cmmarusich@gmail.com>.

* gnu/services/desktop.scm (%desktop-services): Add 'mtp' service via
call to 'simple-service'.

gnu/services/desktop.scm

@@ -40,6 +40,7 @@
   #:use-module (gnu packages xdisorg)
   #:use-module (gnu packages suckless)
   #:use-module (gnu packages linux)
+  #:use-module (gnu packages libusb)
   #:use-module (guix records)
   #:use-module (guix packages)
   #:use-module (guix store)
@@ -753,6 +754,10 @@ with the administrator's password."
          (screen-locker-service slock)
          (screen-locker-service xlockmore "xlock")
 
+         ;; Add udev rules for MTP devices so that non-root users can access
+         ;; them.
+         (simple-service 'mtp udev-service-type (list libmtp))
+
          ;; The D-Bus clique.
          (avahi-service)
          (wicd-service)