util: add parameter parenthesis to __open_proc statement in expr

Fixes small bug:
__open_proc() macro is called from parse_pid_loginuid as:
>	fd = __open_proc(pid, (ignore_noent) ? ENOENT : 0,
>			O_RDONLY, "loginuid");

So, ier parameter is badly expanded with current version:
>	if (__fd < 0 && (errno != (ignore_noent) ? ENOENT : 0)

Which in result does not hide "No such file" error on feature
test (at least with arm-gcc). Not a big deal, tho.

Reported-by: alex vk <avankemp@gmail.com>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>