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/sdbusplus.hpp b/src/sdbusplus.hpp
index 96a8903..02c530d 100644
--- a/src/sdbusplus.hpp
+++ b/src/sdbusplus.hpp
@@ -7,6 +7,7 @@
 #include <sdbusplus/bus/match.hpp>
 #include <sdbusplus/exception.hpp>
 #include <sdbusplus/message.hpp>
+#include <string>
 
 struct Loop;