os: use a different symbol from syscall in mkunixsignals.sh.

Open is more likely to be present than Syscall.

R=rsc
CC=golang-dev
https://golang.org/cl/4645073

src/pkg/os/mkunixsignals.sh

@@ -14,7 +14,7 @@ import (
   "syscall"
 )
 
-var _ = syscall.Syscall  // in case there are zero signals
+var _ = syscall.Open  // in case there are zero signals
 
 const (
 EOH