PHPVersionCheck: Properly check for `xml` extension in PHP 7.2+

PHPVersionCheck: Properly check for `xml` extension in PHP 7.2+

In 7.2, the `utf8_encode` function was moved into core PHP, and is no
longer a part of the `xml` extension. Use a different function to check
for the existence of it.

Change-Id: Ib50c1a59dd9c2a8777b07b2f7a3dd638a55832c6
(cherry picked from commit 9e5785b7204e6a927e7f60ce4cbdeedad8f45818)