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_verify.hpp b/image_verify.hpp
index dcf904d..d52281c 100644
--- a/image_verify.hpp
+++ b/image_verify.hpp
@@ -9,6 +9,7 @@
 
 #include <experimental/filesystem>
 #include <set>
+#include <string>
 
 namespace phosphor
 {