services: mcron: Hide ‘mkdir-p’ from (shepherd support).

This a avoids a Guile warning about the name collision with (guix build
utils).  The warning was introduced by commit
e0e85f2b23.

* gnu/services/mcron.scm (mcron-shepherd-services): Hide ‘mkdir-p’.

gnu/services/mcron.scm

@@ -165,7 +165,7 @@ files."
                             (ice-9 popen) ;for the 'schedule' action
                             (ice-9 rdelim)
                             (ice-9 match)
-                            (shepherd support)    ;for '%user-log-dir'
+                            ((shepherd support) #:hide (mkdir-p)) ;for '%user-log-dir'
                             ,@%default-modules))
                  (start #~(make-forkexec-constructor
                            (list #$(file-append mcron "/bin/mcron")