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: Ic1bf10e15071227582e5738336f632d33705c80b
diff --git a/version.hpp b/version.hpp
index 85dccda..c7d2b03 100644
--- a/version.hpp
+++ b/version.hpp
@@ -7,6 +7,7 @@
 #include "xyz/openbmc_project/Software/Version/server.hpp"
 
 #include <sdbusplus/bus.hpp>
+#include <string>
 
 namespace openpower
 {