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/item_updater.cpp b/item_updater.cpp
index 4e2ca45..9cac8dd 100644
--- a/item_updater.cpp
+++ b/item_updater.cpp
@@ -7,7 +7,7 @@
 #include "version.hpp"
 #include "xyz/openbmc_project/Software/Version/server.hpp"
 
-#include <experimental/filesystem>
+#include <filesystem>
 #include <fstream>
 #include <phosphor-logging/elog-errors.hpp>
 #include <phosphor-logging/elog.hpp>
@@ -32,7 +32,7 @@
 using namespace phosphor::logging;
 using namespace sdbusplus::xyz::openbmc_project::Software::Image::Error;
 using namespace phosphor::software::image;
-namespace fs = std::experimental::filesystem;
+namespace fs = std::filesystem;
 using NotAllowed = sdbusplus::xyz::openbmc_project::Common::Error::NotAllowed;
 
 void ItemUpdater::createActivation(sdbusplus::message::message& msg)