Fix lxc driver build issue on Mac OS X

Docker-DCO-1.1-Signed-off-by: Albert Zhang <zhgwenming@gmail.com> (github: zhgwenming)

daemon/execdriver/lxc/lxc_init_unsupported.go

@@ -2,12 +2,10 @@
 
 package lxc
 
-import "github.com/docker/docker/daemon/execdriver"
-
 func setHostname(hostname string) error {
 	panic("Not supported on darwin")
 }
 
-func finalizeNamespace(args *execdriver.InitArgs) error {
+func finalizeNamespace(args *InitArgs) error {
 	panic("Not supported on darwin")
 }