ibusManager: Use const correctly

Per ES6, a variable declared const should only be valid inside its lexical
scope. Previously, GJS would accept this code, but that will change in the
SpiderMonkey JS engine in the next release of GJS.

https://bugzilla.gnome.org/show_bug.cgi?id=778425

js/misc/ibusManager.js

@@ -6,10 +6,11 @@ const Lang = imports.lang;
 const Mainloop = imports.mainloop;
 const Signals = imports.signals;
 
+let IBusCandidatePopup;
 try {
     var IBus = imports.gi.IBus;
     _checkIBusVersion(1, 5, 2);
-    const IBusCandidatePopup = imports.ui.ibusCandidatePopup;
+    IBusCandidatePopup = imports.ui.ibusCandidatePopup;
 } catch (e) {
     var IBus = null;
     log(e);