tests: Silence a warning

In case the OS does not support epoll and kqueue, we get the warning:

gio/tests/pollable.c: In function ‘test_pollable_unix_nulldev’:
gio/tests/pollable.c:266:7: warning: unused variable ‘fd’ [-Wunused-variable]
  266 |   int fd;

Get rid of it.

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>