Fix compiler warning in Objects/unicodeobject.c (GH-17440)

diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 7776019..1ec2acc 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -15950,7 +15950,7 @@
     interp->fs_codec.encoding = NULL;
     PyMem_RawFree(interp->fs_codec.errors);
     interp->fs_codec.errors = NULL;
-    interp->config.filesystem_errors = _Py_ERROR_UNKNOWN;
+    interp->config.filesystem_errors = (wchar_t *)_Py_ERROR_UNKNOWN;
 }