theme: add bottom badding for login dialog hint

Since we're getting rid of the top padding, we need to add what we
take away to the bottom padding to keep the size the same.

https://bugzilla.gnome.org/show_bug.cgi?id=706670

data/theme/gnome-shell.css

@@ -2445,6 +2445,7 @@ StScrollBar StButton#vhandle:active {
 
 .login-dialog-message-hint {
     padding-top: 0px;
+    padding-bottom: 20px;
 }
 
 .user-widget-label {