Fix fseek_ and ftell_ on Win32

- on Windows fseek_ returned always "the end" of a stream, added "fflush" to make it work again


More details can be found here:
http://www.tek-tips.com/viewthread.cfm?qid=1234654
https://code.google.com/p/mp4v2/source/browse/trunk/libplatform/io/File_win32.cpp?r=295

So it seems the correct way of fixing it.