Update ext/session spl dependency (#14410)

The spl dependency is configured so the spl_autoload_register is
available when session_start() is used, meaning the spl extension needs
to be loaded before the session. It is marked as optional to be more
explicit as spl is not directly used nor required in the session
extension.

ext/session/session.c

@@ -2998,7 +2998,7 @@ static PHP_MINFO_FUNCTION(session) /* {{{ */
 /* }}} */
 
 static const zend_module_dep session_deps[] = { /* {{{ */
-	ZEND_MOD_REQUIRED("spl")
+	ZEND_MOD_OPTIONAL("spl")
 	ZEND_MOD_END
 };
 /* }}} */