Windows: No ZFS graphdriver

Signed-off-by: John Howard <jhoward@microsoft.com>

daemon/daemon_zfs.go

@@ -1,4 +1,4 @@
-// +build !exclude_graphdriver_zfs
+// +build !exclude_graphdriver_zfs,linux
 
 package daemon
 

daemon/graphdriver/zfs/zfs.go

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

daemon/graphdriver/zfs/zfs_test.go

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

daemon/graphdriver/zfs/zfs_unsupported.go

@@ -0,0 +1,3 @@
+// +build !linux
+
+package zfs