Add missing pthread.h include to logging.h

Change-Id: I986b3e978616510ed92903e192222be513297284
BUG: 920369
Signed-off-by: Justin Clift <jclift@redhat.com>
Reviewed-on: http://review.gluster.org/4653
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Kaleb KEITHLEY <kkeithle@redhat.com>