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_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