bpo-43394: Fix -Wstrict-prototypes warnings (GH-24737)

diff --git a/Modules/_zoneinfo.c b/Modules/_zoneinfo.c
index 4726b82..755442f 100644
--- a/Modules/_zoneinfo.c
+++ b/Modules/_zoneinfo.c
@@ -2608,7 +2608,7 @@ static PyTypeObject PyZoneInfo_ZoneInfoType = {
 // Specify the _zoneinfo module
 static PyMethodDef module_methods[] = {{NULL, NULL}};
 static void
-module_free()
+module_free(void)
 {
     Py_XDECREF(_tzpath_find_tzfile);
     _tzpath_find_tzfile = NULL;