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: Ie93f251b00db4d4a543b361262fad2201923e2c5
diff --git a/src/format.hpp b/src/format.hpp
index 32a85f8..fe8565b 100644
--- a/src/format.hpp
+++ b/src/format.hpp
@@ -2,6 +2,8 @@
 
 #include "data_types.hpp"
 
+#include <string>
+
 namespace phosphor
 {
 namespace dbus