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: Ie93f251b00db4d4a543b361262fad2201923e2c5
diff --git a/src/method.hpp b/src/method.hpp
index f8f8f51..1399531 100644
--- a/src/method.hpp
+++ b/src/method.hpp
@@ -4,6 +4,7 @@
 
 #include <experimental/tuple>
 #include <phosphor-logging/log.hpp>
+#include <string>
 
 namespace phosphor
 {