graph/load: add build tags to fix make cross

Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com>

graph/load.go

@@ -1,3 +1,5 @@
+// +build linux
+
 package graph
 
 import (

graph/load_unsupported.go

@@ -0,0 +1,11 @@
+// +build !linux
+
+package graph
+
+import (
+	"github.com/docker/docker/engine"
+)
+
+func (s *TagStore) CmdLoad(job *engine.Job) engine.Status {
+	return job.Errorf("CmdLoad is not supported on this platform")
+}