bpo-33012: Fix invalid function cast warnings with gcc 8 for METH_NOARGS. (GH-6030)

METH_NOARGS functions need only a single argument but they are cast
into a PyCFunction, which takes two arguments.  This triggers an
invalid function cast warning in gcc8 due to the argument mismatch.
Fix this by adding a dummy unused argument.
diff --git a/PC/bdist_wininst/install.c b/PC/bdist_wininst/install.c
index 04323eb..6d01ad5 100644
--- a/PC/bdist_wininst/install.c
+++ b/PC/bdist_wininst/install.c
@@ -631,19 +631,18 @@
     return g_Py_BuildValue("i", rc);
 }
 
-static PyObject *GetRootHKey(PyObject *self)
+static PyObject *GetRootHKey(PyObject *self, PyObject *Py_UNUSED(ignored))
 {
     return g_PyLong_FromVoidPtr(hkey_root);
 }
 
 #define METH_VARARGS 0x0001
 #define METH_NOARGS   0x0004
-typedef PyObject *(*PyCFunction)(PyObject *, PyObject *);
 
 PyMethodDef meth[] = {
     {"create_shortcut", CreateShortcut, METH_VARARGS, NULL},
     {"get_special_folder_path", GetSpecialFolderPath, METH_VARARGS, NULL},
-    {"get_root_hkey", (PyCFunction)GetRootHKey, METH_NOARGS, NULL},
+    {"get_root_hkey", GetRootHKey, METH_NOARGS, NULL},
     {"file_created", FileCreated, METH_VARARGS, NULL},
     {"directory_created", DirectoryCreated, METH_VARARGS, NULL},
     {"message_box", PyMessageBox, METH_VARARGS, NULL},