string: ensure string included

Latest upstream yocto appears to have removed a free include of string.
This is causing compile failures for files which do not include it
properly.

Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
Change-Id: I457b7ca46bd010a9c5b363495d43a9b38e430bcf
diff --git a/item_updater.hpp b/item_updater.hpp
index 3f0530f..313c568 100644
--- a/item_updater.hpp
+++ b/item_updater.hpp
@@ -6,6 +6,7 @@
 #include "xyz/openbmc_project/Collection/DeleteAll/server.hpp"
 
 #include <sdbusplus/server.hpp>
+#include <string>
 #include <xyz/openbmc_project/Association/Definitions/server.hpp>
 #include <xyz/openbmc_project/Common/FactoryReset/server.hpp>
 #include <xyz/openbmc_project/Control/FieldMode/server.hpp>