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/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