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: I76b1d73d045f98bd457c155086e000a430134918
diff --git a/sol/sol_manager.hpp b/sol/sol_manager.hpp
index caeaa39..6a5887b 100644
--- a/sol/sol_manager.hpp
+++ b/sol/sol_manager.hpp
@@ -8,6 +8,7 @@
 #include <boost/asio/local/stream_protocol.hpp>
 #include <map>
 #include <memory>
+#include <string>
 
 namespace sol
 {