Fix a compile warning in selectmodule.c. (GH-16617)

diff --git a/Modules/selectmodule.c b/Modules/selectmodule.c
index 98da7d5..79cc1b2 100644
--- a/Modules/selectmodule.c
+++ b/Modules/selectmodule.c
@@ -2362,7 +2362,7 @@
 
 static PyType_Slot kqueue_queue_Type_slots[] = {
     {Py_tp_dealloc, kqueue_queue_dealloc},
-    {Py_tp_doc, select_kqueue__doc__},
+    {Py_tp_doc, (void*)select_kqueue__doc__},
     {Py_tp_getset, kqueue_queue_getsetlist},
     {Py_tp_methods, kqueue_queue_methods},
     {Py_tp_new, select_kqueue},