gnu: gtk+: Add CUPS print back-end.

* gnu/packages/gtk.scm (gtk+)[inputs]: Add CUPS.

gnu/packages/gtk.scm

 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2013 Andreas Enge <andreas@enge.fr>
-;;; Copyright © 2013, 2014 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2013, 2014, 2015 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2014, 2015 Mark H Weaver <mhw@netris.org>
 ;;; Copyright © 2014 Eric Bavier <bavier@member.fsf.org>
 ;;;
@@ -38,6 +38,7 @@ (define-module (gnu packages gtk)
   #:use-module (gnu packages pkg-config)
   #:use-module (gnu packages python)
   #:use-module (gnu packages guile)
+  #:use-module (gnu packages cups)
   #:use-module (gnu packages xml)
   #:use-module (gnu packages xorg)
   #:use-module (gnu packages xdisorg))
@@ -411,7 +412,8 @@ (define-public gtk+
       ("libxdamage" ,libxdamage)
       ("pango" ,pango)))
    (inputs
-    `(("libxml2" ,libxml2)))
+    `(("libxml2" ,libxml2)
+      ("cups" ,cups)))                            ;for printing support
    (native-inputs
     `(("perl" ,perl)
       ("glib" ,glib "bin")