Reintroduce unused cache_size in mkcache_bytes

Although this is unused, we shouldn't change more than is necessary to get this
to build on Windows/MSVC.

src/python/core.c

@@ -21,6 +21,7 @@
 static PyObject *
 mkcache_bytes(PyObject *self, PyObject *args) {
  unsigned long block_number;
+ unsigned long cache_size;
 
  if (!PyArg_ParseTuple(args, "k", &block_number))
  return 0;