Update payment_address.php

Fixes an Error when adding a new address while checking out. (I don't know if this error is caused because I imported a backup from an older version of OpenCart, or if this is a normal issue, but this fixed it either way)

The error was: (Which is caused by the missing addActivity function found in file in the added line)
SyntaxError: JSON.parse: unexpected character at line 1 column 1 of the JSON data
OK

<b>Fatal error</b>:  Call to a member function addActivity() on a non-object in <b>Z:\xampp\htdocs\printstore2\catalog\controller\checkout\payment_address.php</b> on line <b>182</b><br />


![error](https://cloud.githubusercontent.com/assets/7613391/3006470/21b0e6d2-de2e-11e3-8c5b-573fc3698b6c.jpg)