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/image_manager.hpp b/image_manager.hpp
index 8498c77..4f2b206 100644
--- a/image_manager.hpp
+++ b/image_manager.hpp
@@ -2,6 +2,7 @@
 #include "version.hpp"
 
 #include <sdbusplus/server.hpp>
+#include <string>
 
 namespace phosphor
 {