add missing gmock header: read|write interface mock

Change-Id: I8633300650d1f40d9d5b880ebee3be51236db3c5
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/test/readinterface_mock.hpp b/test/readinterface_mock.hpp
index 0735559..b8ab5ac 100644
--- a/test/readinterface_mock.hpp
+++ b/test/readinterface_mock.hpp
@@ -2,6 +2,8 @@
 
 #include "interfaces.hpp"
 
+#include <gmock/gmock.h>
+
 class ReadInterfaceMock : public ReadInterface
 {
     public: