environment: Define N_() globally

Commit 1291bcd0c8 implemented it for dateMenu, but the function
is already used in screenShield as well. Just add it globally as
we do for other standard gettext "macros".

js/ui/dateMenu.js

@@ -19,8 +19,6 @@ const PanelMenu = imports.ui.panelMenu;
 const PopupMenu = imports.ui.popupMenu;
 const Calendar = imports.ui.calendar;
 
-const N_ = function(s) { return s; };
-
 function _onVertSepRepaint(area) {
     let cr = area.get_context();
     let themeNode = area.get_theme_node();

js/ui/environment.js

@@ -77,6 +77,7 @@ function init() {
     window._ = Gettext.gettext;
     window.C_ = Gettext.pgettext;
     window.ngettext = Gettext.ngettext;
+    window.N_ = function(s) { return s; };
 
     // Miscellaneous monkeypatching
     _patchContainerClass(St.BoxLayout);