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

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang