gdm: make entry reactive when asking user question

We disable the entry after a user answers a question while we
process it, but we don't reactivate it later if asked another
one.

This commit makes sure the entry is always reactive when we
are waiting for an answer from the user.

https://bugzilla.gnome.org/show_bug.cgi?id=691806

js/gdm/loginDialog.js

@@ -881,6 +881,7 @@ const LoginDialog = new Lang.Class({
     _askQuestion: function(verifier, serviceName, question, passwordChar) {
         this._promptLabel.set_text(question);
 
+        this._updateSensitivity(true);
         this._promptEntry.set_text('');
         this._promptEntry.clutter_text.set_password_char(passwordChar);