Don't build portallocator on Windows

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

libnetwork/portallocator/portallocator.go

@@ -1,3 +1,5 @@
+// +build !windows
+
 package portallocator
 
 import (

libnetwork/portallocator/portallocator_windows.go

@@ -0,0 +1 @@
+package portallocator