Fix compile error with g++ 7.3

Get the below error message when compiling with g++ 7.3:
selutility.hpp:131:6: error: 'chrono' in namespace 'std' does not name a type

Change the include order to fix this issue

Change-Id: Ieaac302dd2e411a71736cbbe3f92431e30f3f5ea
Signed-off-by: Yong Li <yong.b.li@linux.intel.com>
diff --git a/selutility.hpp b/selutility.hpp
index 5262d82..4e2855a 100644
--- a/selutility.hpp
+++ b/selutility.hpp
@@ -2,6 +2,7 @@
 
 #include "types.hpp"
 
+#include <chrono>
 #include <cstdint>
 #include <sdbusplus/server.hpp>