Use fseeko64 and related functions; mingw32 doesn't know about fseeko;

I'm compiling idevicerestore on mingw32. 
The fseeko and ftello functions aren't defined natively on mingw32, but fseeko64 and ftello64 are. 
In this patch, I've replaced the calls to fopen, fseeko and ftello to fopen64, fseeko64 and ftello64, allowing compilation on mingw32 to succeed.