Fixed: compile error on systems where _fseeki64 isn't defined

As .nes files don't really get greater than 2 GiB, we might be able to solve this compile error by swapping out _fseeki64() for fseek().

Another possibility is to isolate each system that doesn't support _fseeki64. For example, 
ac4b7eda63ccd01e80b9618cbe91859921b0f732. However, this doesn't seem like a scalable solution.