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/download_manager.hpp b/download_manager.hpp
index edafaab..b53fc2b 100644
--- a/download_manager.hpp
+++ b/download_manager.hpp
@@ -3,6 +3,7 @@
 #include "xyz/openbmc_project/Common/TFTP/server.hpp"
 
 #include <sdbusplus/bus.hpp>
+#include <string>
 
 namespace phosphor
 {