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: I6a0afb2e01d0ed0630a28f8798a16a66b0142cab
diff --git a/manager.hpp b/manager.hpp
index b0446b6..eff9e10 100644
--- a/manager.hpp
+++ b/manager.hpp
@@ -9,6 +9,7 @@
 #include <experimental/filesystem>
 #include <map>
 #include <sdbusplus/bus.hpp>
+#include <string>
 #ifdef USE_POLICY_INTERFACE
 #include "policy_table.hpp"
 #endif