Use std::filesystem instead of std::experimental

std::filesystem is fully supported in c++17, no need to keep
the experimental around anymore.

Note that the path append behavior changed, so appending an
absolute path replaces the path instead of appending, therefore
need to use .relative_path().

Tested: Did a code update on witherspoon and verified the sync
        path was correct.

Change-Id: Iffe42cf0ebd5537920c8fdbf9edcec640ab9c724
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
diff --git a/serialize.cpp b/serialize.cpp
index 31ac971..f5aeb11 100644
--- a/serialize.cpp
+++ b/serialize.cpp
@@ -3,7 +3,7 @@
 #include "serialize.hpp"
 
 #include <cereal/archives/json.hpp>
-#include <experimental/filesystem>
+#include <filesystem>
 #include <fstream>
 #include <phosphor-logging/log.hpp>
 #include <sdbusplus/server.hpp>
@@ -16,7 +16,7 @@
 {
 
 using namespace phosphor::logging;
-namespace fs = std::experimental::filesystem;
+namespace fs = std::filesystem;
 
 const std::string priorityName = "priority";
 const std::string purposeName = "purpose";