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: Ic1bf10e15071227582e5738336f632d33705c80b
diff --git a/image_verify.hpp b/image_verify.hpp
index 0aff4b8..d4f0930 100644
--- a/image_verify.hpp
+++ b/image_verify.hpp
@@ -9,6 +9,7 @@
 
 #include <experimental/filesystem>
 #include <set>
+#include <string>
 
 namespace openpower
 {