cleanup: Use filesystem instead of experimental

With Thud(Yocto 2.6) bumping gcc version, std::filesystem
is available.

Change-Id: I1837df14e52be3e2007b941c3f9c0d3efc687027
Signed-off-by: Kun Yi <kunyi731@gmail.com>
diff --git a/serialize.hpp b/serialize.hpp
index e724919..4028899 100644
--- a/serialize.hpp
+++ b/serialize.hpp
@@ -3,7 +3,7 @@
 #include "config.h"
 
 #include <cereal/archives/json.hpp>
-#include <experimental/filesystem>
+#include <filesystem>
 #include <fstream>
 #include <phosphor-logging/log.hpp>
 
@@ -14,7 +14,7 @@
 namespace manager
 {
 
-namespace fs = std::experimental::filesystem;
+namespace fs = std::filesystem;
 
 struct SerialOps
 {