This patch fixes a build failure[1], by adding the missing semaphore.h include

References:
[1] http://kisskb.ellerman.id.au/kisskb/buildresult/2234322/Signed-off-by: NPeter Huewe <peterhuewe@gmx.de>
Signed-off-by: NSamuel Ortiz <sameo@linux.intel.com>