runtime: add missing nacl build tag for lock_sema.c

LGTM=dave
R=rsc, dave
CC=golang-codereviews
https://golang.org/cl/72240045

src/pkg/runtime/lock_sema.c

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build darwin netbsd openbsd plan9 solaris windows
+// +build darwin nacl netbsd openbsd plan9 solaris windows
 
 #include "runtime.h"
 #include "stack.h"