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/auth_algo.hpp b/auth_algo.hpp
index 6ed0b47..5bf1a1e 100644
--- a/auth_algo.hpp
+++ b/auth_algo.hpp
@@ -4,6 +4,7 @@
 #include "integrity_algo.hpp"
 
 #include <array>
+#include <string>
 #include <vector>
 
 namespace cipher
diff --git a/sd_event_loop.hpp b/sd_event_loop.hpp
index 9729078..60b4434 100644
--- a/sd_event_loop.hpp
+++ b/sd_event_loop.hpp
@@ -8,6 +8,7 @@
 #include <chrono>
 #include <map>
 #include <sdbusplus/asio/connection.hpp>
+#include <string>
 
 namespace ipmi
 {
diff --git a/sessions_manager.hpp b/sessions_manager.hpp
index f9c00b5..ed3db49 100644
--- a/sessions_manager.hpp
+++ b/sessions_manager.hpp
@@ -7,6 +7,7 @@
 #include <map>
 #include <memory>
 #include <mutex>
+#include <string>
 
 namespace session
 {
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
 {