Fix _ErrorLog not iterable

Fix the following mypy error:
```
lib/galaxy/tool_util/linters/xsd.py:28: error: "_ErrorLog" has no attribute
"__iter__" (not iterable)  [attr-defined]
                for error in xsd.error_log:
                             ^~~~~~~~~~~~~
```

xref: https://github.com/galaxyproject/galaxy/pull/17081/files#r1490662640

lxml-stubs/etree.pyi

@@ -577,7 +577,12 @@ def tostring(
     inclusive_ns_prefixes: Any = ...,
 ) -> _AnyStr: ...
 
-class _ErrorLog: ...
+class _LogEntry:
+    path: Optional[str]
+
+class _ErrorLog:
+    def __iter__(self) -> Iterator["_LogEntry"]: ...
+
 class Error(Exception): ...
 
 class LxmlError(Error):