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/callout.hpp b/callout.hpp
index b860b9a..32351d5 100644
--- a/callout.hpp
+++ b/callout.hpp
@@ -4,6 +4,7 @@
 #include "interfaces.hpp"
 
 #include <experimental/filesystem>
+#include <string>
 
 namespace ibm
 {