gnu: sddm: Refer to the 'shadow' input.

This change was suggested by Ludovic Courtès <ludo@gnu.org> to allow
users to replace the shadow input in the package.

* gnu/packages/display-manager.scm (sddm)[arguments]: In configure-flags
refer to the shadow package that is an input.

gnu/packages/display-managers.scm

@@ -108,7 +108,7 @@ (define-public sddm
             ;; Set path to /etc/login.defs.
             ;; An alternative would be to use -DUID_MIN and -DUID_MAX.
             (string-append "-DLOGIN_DEFS_PATH="
-                           #$shadow
+                           #$(this-package-input "shadow")
                            "/etc/login.defs")
             (string-append "-DQT_IMPORTS_DIR="
                            #$output "/lib/qt5/qml")