maint: Add gdk-pixbuf to native Hurd manifest.

Gdk-pixbuf is a dependency for `guix system reconfigure' and currently also
needs an unreleased glibc patch in order to build.

* etc/hurd-manifest.scm: Add gdk-pixbuf to manifest.

etc/hurd-manifest.scm

@@ -31,7 +31,7 @@
              (srfi srfi-1))
 
 (use-package-modules
- base bootloaders commencement compression file gawk gdb gettext guile
+ base bootloaders commencement compression file gawk gdb gettext gtk guile
  hurd less m4 package-management python ssh version-control)
 
 (define (input->package input)
@@ -72,6 +72,9 @@
   ;; system
   grub-minimal grub
 
+  ;; system reconfigure
+  gdk-pixbuf
+
   (append
    guix-dependencies
    %base-packages/hurd)))