Change wl_os_dupfd_cloexec minfd to be int

The fcntl() argument is defined to be an int and not a long. This does not
matter on most architectures since the value is passed in registers, but
it causes issues on big-endian architectures that pass variadic arguments
on the stack.

Signed-off-by: Alex Richardson <Alexander.Richardson@cl.cam.ac.uk>
Reviewed-by: Simon Ser <contact@emersion.fr>