Fix not thread safe RelaxNG invocations in ZTS builds

Author: ndossche

NEWS

@@ -20,7 +20,7 @@ PHP                                                                        NEWS
     filter is provided). (Girgias)
 
 - LibXML:
-  . Fix not thread safe xmlSchemaParse calls. (SpencerMalone)
+  . Fix not thread safe schema/relaxng calls. (SpencerMalone, nielsdos)
 
 - Opcache:
   . Fixed bug GH-20081 (access to uninitialized vars in preload_load()).

ext/libxml/libxml.c

@@ -938,6 +938,9 @@ PHP_LIBXML_API void php_libxml_initialize(void)
 		xmlInitParser();
 #ifdef LIBXML_SCHEMAS_ENABLED
 		xmlSchemaInitTypes();
+#endif
+#ifdef LIBXML_RELAXNG_ENABLED
+		xmlRelaxNGInitTypes();
 #endif
 		ZEND_IGNORE_LEAKS_END();