string: ensure string included when used

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: I8df4c564af2c8e8294cdcb3e74f9d7cd106d4ec8
diff --git a/ledlayout.hpp b/ledlayout.hpp
index 90b333c..f12e0b5 100644
--- a/ledlayout.hpp
+++ b/ledlayout.hpp
@@ -2,6 +2,7 @@
 
 #include <map>
 #include <set>
+#include <string>
 
 namespace phosphor
 {