Windows: Allow compilation (again)

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

libnetwork/sandbox/namespace_unsupported.go

@@ -0,0 +1,8 @@
+// +build !linux
+
+package sandbox
+
+// GC triggers garbage collection of namespace path right away
+// and waits for it.
+func GC() {
+}