Add phone number utils to background page

Quickfix.
Probably it shouldn't actually be verifying any phone numbers.

background.html

@@ -28,6 +28,7 @@
 	<script type="text/javascript" src="js-deps/libphonenumber_api-compiled.js"></script>
 
 	<script type="text/javascript" src="js/helpers.js"></script>
+  <script type="text/javascript" src="js/libphonenumber-util.js"></script>
 	<script type="text/javascript" src="js/webcrypto.js"></script>
 	<script type="text/javascript" src="js/crypto.js"></script>
 	<script type="text/javascript" src="js/models/messages.js"></script>