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/mslverify/util.hpp b/mslverify/util.hpp
index ae83006..2d20392 100644
--- a/mslverify/util.hpp
+++ b/mslverify/util.hpp
@@ -5,6 +5,7 @@
 #include <phosphor-logging/log.hpp>
 #include <sdbusplus/bus.hpp>
 #include <sdbusplus/message.hpp>
+#include <string>
 #include <xyz/openbmc_project/Common/error.hpp>
 
 namespace util