cups: make string non-literals translatable again

Notice that C_ just takes string literals as arguments.

https://bugzilla.gnome.org/show_bug.cgi?id=764585

modules/printbackends/cups/gtkprintbackendcups.c

@@ -4442,7 +4442,7 @@ get_ipp_choice_translation_string (gint  index,
             g_assert_not_reached ();
         }
 
-        translation = NC_("output-bin", string);
+        translation = g_dpgettext2 (GETTEXT_PACKAGE, "output-bin", string);
     }
 
   return translation;