Avoid pointer/integer mismatch.

Index: curve25519module.c
--- curve25519module.c.orig
+++ curve25519module.c
@@ -158,7 +158,7 @@ curve25519_functions[] = {
         PyModuleDef_HEAD_INIT,
         "axolotl_curve25519",
         NULL,
-        NULL,
+        0,
         curve25519_functions,
     };