fix build on Ruby 3

[tenderlove]

Hi,

This gem doesn't build on Ruby 3. If I try to build on Ruby 3.0, I get the following errors:

../../../../ext/termios.c:311:5: error: use of undeclared identifier 'OpenFile'
    OpenFile *fptr;
    ^
../../../../ext/termios.c:311:15: error: use of undeclared identifier 'fptr'
    OpenFile *fptr;
              ^
../../../../ext/termios.c:314:21: error: use of undeclared identifier 'fptr'
    GetOpenFile(io, fptr);
                    ^

The patch in this PR makes the gem build again.

Thanks!

[arika]

Thank you 😃