runtime/race: do not include pthread.h Fixes issue 4721.
runtime/race: do not include pthread.h
Fixes issue 4721.

R=alex.brainman, minux.ma
CC=golang-dev
https://codereview.appspot.com/7275048
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go