empty: fix compile with toolchain w/o threads

As found via autobuilders and discussed on the list, the
ifdef is wrong, as empty uses SysV semaphores and not POSIX
semaphores.

Tested with glibc/uclibc-ng builds. And with uclibc-ng without
threads.

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>