gettime: Fix compilation on non-Linux with pthread_getaffinity_np()

874d55e50c("os/os-linux: add pthread CPU affinity helper") and a few
commits after that broke compilation on non-Linux platforms which support
pthread_getaffinity_np().

Define fio_get_thread_affinity() on non-Linux platforms, and make gettime
test FIO_HAVE_GET_THREAD_AFFINITY which may or may not depend on pthread.
FIO_HAVE_GET_THREAD_AFFINITY is currently not defined on Windows.

Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>