Restrict sigev_notify_thread_id macro definition to Linux

The `#define sigev_notify_thread_id _sigev_un._tid` macro differs on
FreeBSD. This patch uses ifdefs to stop the compiler from overriding
FreeBSD's existing definition.

Signed-off-by: Jake Freeland <jfree@freebsd.org>
Reviewed-by: Kamil Konieczny <kamil.konieczny@linux.intel.com>