Fix strict standards issue-- DOMDocument isn't static

When I imported my OPML under PHP 5.4 I got:

Strict Standards: Non-static method DOMDocument::load() should not be called statically, assuming $this from incompatible context in /path/to/ttrss/classes/opml.php on line 465

This patch changes the call to be non-static.  While the DOM module explicitly supports both static and non-static calling conventions, more recent PHP's have deprecated that generally.