runtime: add missing nacl build tag for lock_sema.c
runtime: add missing nacl build tag for lock_sema.c

LGTM=dave
R=rsc, dave
CC=golang-codereviews
https://codereview.appspot.com/72240045

firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old