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: Ic1bf10e15071227582e5738336f632d33705c80b
diff --git a/static/activation_static.hpp b/static/activation_static.hpp
index 258a612..8cf563c 100644
--- a/static/activation_static.hpp
+++ b/static/activation_static.hpp
@@ -3,6 +3,7 @@
 #include "activation.hpp"
 
 #include <filesystem>
+#include <string>
 
 namespace openpower
 {