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: I6a0afb2e01d0ed0630a28f8798a16a66b0142cab
diff --git a/callout.hpp b/callout.hpp
index b860b9a..32351d5 100644
--- a/callout.hpp
+++ b/callout.hpp
@@ -4,6 +4,7 @@
 #include "interfaces.hpp"
 
 #include <experimental/filesystem>
+#include <string>
 
 namespace ibm
 {
diff --git a/manager.hpp b/manager.hpp
index b0446b6..eff9e10 100644
--- a/manager.hpp
+++ b/manager.hpp
@@ -9,6 +9,7 @@
 #include <experimental/filesystem>
 #include <map>
 #include <sdbusplus/bus.hpp>
+#include <string>
 #ifdef USE_POLICY_INTERFACE
 #include "policy_table.hpp"
 #endif
diff --git a/policy_find.hpp b/policy_find.hpp
index 5e45a5b..4ce9935 100644
--- a/policy_find.hpp
+++ b/policy_find.hpp
@@ -3,6 +3,8 @@
 #include "dbus.hpp"
 #include "policy_table.hpp"
 
+#include <string>
+
 namespace ibm
 {
 namespace logging
diff --git a/policy_table.hpp b/policy_table.hpp
index d6bf307..013578f 100644
--- a/policy_table.hpp
+++ b/policy_table.hpp
@@ -4,6 +4,7 @@
 
 #include <map>
 #include <optional>
+#include <string>
 #include <vector>
 
 namespace ibm