pid/zone: split zone interface into its own header

Signed-off-by: Patrick Venture <venture@google.com>
Change-Id: I8f516353ddf7777ec750549e748c96afa211ea6e
diff --git a/test/zone_mock.hpp b/test/zone_mock.hpp
index 541f95c..f5044f8 100644
--- a/test/zone_mock.hpp
+++ b/test/zone_mock.hpp
@@ -1,6 +1,6 @@
 #pragma once
 
-#include "pid/zone.hpp"
+#include "pid/zone_interface.hpp"
 
 #include <string>