kinect_upload_fw: fix reading the firmware file on Windows

In MS Windows fread() behaves differently than on UNIX systems when
reading binary files if we do not specify "b" in the mode when opening
the file.

Quoting FOPEN(3) man page:

 ... the 'b' is ignored on all POSIX conforming systems, including
 Linux. (Other systems may treat text files and binary files
 differently, and adding the 'b' may be a good idea if you do I/O to a
 binary file and expect that your program may be ported to non-UNIX
 environments.)