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/group.hpp b/group.hpp
index 4e0a283..fead647 100644
--- a/group.hpp
+++ b/group.hpp
@@ -4,7 +4,9 @@
 
 #include <sdbusplus/bus.hpp>
 #include <sdbusplus/server/object.hpp>
+#include <string>
 #include <xyz/openbmc_project/Led/Group/server.hpp>
+
 namespace phosphor
 {
 namespace led