unix: fix -Wsign-compare warning in tty.c
unix: fix -Wsign-compare warning in tty.c

The mode argument is an enum now and the signedness of an enum is
implementation-defined when it doesn't have negative members.

Cast it to int in the comparison to tty->mode because the latter is
still an int.

PR: https://github.com/libuv/libuv/pull/134
Reviewed-by: Saúl Ibarra Corretgé <8a7991f6950ea189abf159d6ec98d2fcc13dc5f6@gmail.com>

unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso