Define doXInclude for XSLTProcessor, and test the property

This was added in 8d1427d, but never defined on the stub.
It was more or less fine when dynamic properties were not deprecated,
but now they throw a deprecation warning. To fix it, define on the stub.
This should also help discoverability of the functionality.

Author: nielsdos

ext/xsl/php_xsl.stub.php

@@ -71,6 +71,9 @@
 
 class XSLTProcessor
 {
+    /** @var bool */
+    public $doXInclude;
+
     /**
      * @param DOMDocument|SimpleXMLElement $stylesheet
      * @tentative-return-type

ext/xsl/php_xsl_arginfo.h

@@ -0,0 +1,4 @@
+<?xml version="1.0"?>
+<data>
+    <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="xincluded.xml" parse="xml"/>
+</data>

ext/xsl/tests/xinclude/data.xml

@@ -0,0 +1,41 @@
+--TEST--
+doXInclude
+--EXTENSIONS--
+xsl
+dom
+--FILE--
+<?php
+
+chdir(__DIR__);
+
+$xml = new DOMDocument;
+$xml->loadXML('<?xml version="1.0"?><root/>');
+
+$xsl = new DOMDocument;
+$xsl->loadXML(<<<XML
+<?xml version="1.0"?>
+<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">
+    <xsl:template match="/root">
+        <container>
+            <xsl:value-of select="document('data.xml')/data/content"/>
+        </container>
+    </xsl:template>
+</xsl:stylesheet>
+XML);
+
+$xslt = new XSLTProcessor;
+$xslt->doXInclude = true;
+$xslt->importStylesheet($xsl);
+echo $xslt->transformToXml($xml);
+
+$xslt = new XSLTProcessor;
+$xslt->doXInclude = false;
+$xslt->importStylesheet($xsl);
+echo $xslt->transformToXml($xml);
+
+?>
+--EXPECT--
+<?xml version="1.0"?>
+<container>This is sample content</container>
+<?xml version="1.0"?>
+<container/>

ext/xsl/tests/xinclude/xinclude.phpt

@@ -0,0 +1,2 @@
+<?xml version="1.0"?>
+<content>This is sample content</content>