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

pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go