gnu: services: Install policies for polkit service.

* gnu/services/desktop.scm (polkit-etc-files): Add the polkit package
  itself to the list of packages to scan for .policy files.  This add
  support for pkexec.

gnu/services/desktop.scm

@@ -378,7 +378,7 @@ (define (polkit-directory packages)
 (define polkit-etc-files
   (match-lambda
     (($ <polkit-configuration> polkit packages)
-     `(("polkit-1" ,(polkit-directory packages))))))
+     `(("polkit-1" ,(polkit-directory (cons polkit packages)))))))
 
 (define polkit-setuid-programs
   (match-lambda