Merge pull request #470 from improbable-io/bug-darwin-build-2

Fix build with bazel on OSX attempt 2 (add stub for host_cgo.go)

host/host_cgo.go

@@ -0,0 +1,5 @@
+// +build cgo
+
+package host
+
+import "C"

host/host_darwin_cgo.go

@@ -4,7 +4,7 @@
 package host
 
 // #cgo LDFLAGS: -framework IOKit
-// #include "include/smc.c"
+// #include "smc.h"
 import "C"
 import "context"