Followup to bug 846524 to fix --enable-extensions on comm-central, rs=Callek.

mail/app.mozbuild

@@ -17,7 +17,7 @@ if not CONFIG['COMM_BUILD']:
         include('/toolkit/toolkit.mozbuild')
 
     if CONFIG['MOZ_EXTENSIONS']:
-        add_tier_dir('app', 'toolkit/extensions')
+        add_tier_dir('app', 'extensions')
 
 else:
     bridge_reldir = ''