string: ensure string included when used

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: I8df4c564af2c8e8294cdcb3e74f9d7cd106d4ec8
diff --git a/fault-monitor/fru-fault-monitor.hpp b/fault-monitor/fru-fault-monitor.hpp
index 54eb655..d8ec721 100644
--- a/fault-monitor/fru-fault-monitor.hpp
+++ b/fault-monitor/fru-fault-monitor.hpp
@@ -4,6 +4,7 @@
 
 #include <sdbusplus/bus.hpp>
 #include <sdbusplus/server.hpp>
+#include <string>
 
 namespace phosphor
 {