string: ensure string included when used

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: I8df4c564af2c8e8294cdcb3e74f9d7cd106d4ec8
diff --git a/manager.hpp b/manager.hpp
index 85319fe..2c79a2f 100644
--- a/manager.hpp
+++ b/manager.hpp
@@ -5,6 +5,7 @@
 #include <map>
 #include <sdbusplus/bus.hpp>
 #include <set>
+#include <string>
 
 namespace phosphor
 {