Fix implicit conversion warning

I understand this file is originally part of MiniZip, but I think it would still be nice to fix the warning.

Duplicate of https://github.com/soffes/ssziparchive/issues/82

```
SSZipArchive/minizip/ioapi.c:195:33: Implicit conversion loses integer precision: 'ZPOS64_T' (aka 'unsigned long long') to 'long'
```