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/event_serialize.hpp b/src/event_serialize.hpp
index d331950..bb6f22f 100644
--- a/src/event_serialize.hpp
+++ b/src/event_serialize.hpp
@@ -5,6 +5,7 @@
 #include "event_entry.hpp"
 
 #include <experimental/filesystem>
+#include <string>
 
 namespace phosphor
 {