Only use O_BINARY on Win32

Use of O_BINARY fails on Linux systems:

    gcc -O2 -g -Wall -W `pkg-config --cflags librtlsdr`  -c dump1090.c
    dump1090.c: In function ‘main’:
    dump1090.c:713:65: error: ‘O_BINARY’ undeclared (first use in this function)
             } else if ((Modes.fd = open(Modes.filename, (O_RDONLY | O_BINARY))) == -1) {
                                                                     ^