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/download_manager.cpp b/download_manager.cpp
index 6357ee8..a3811dd 100644
--- a/download_manager.cpp
+++ b/download_manager.cpp
@@ -8,7 +8,7 @@
 #include <unistd.h>
 
 #include <algorithm>
-#include <experimental/filesystem>
+#include <filesystem>
 #include <iostream>
 #include <phosphor-logging/elog-errors.hpp>
 #include <phosphor-logging/elog.hpp>
@@ -24,7 +24,7 @@
 
 using namespace sdbusplus::xyz::openbmc_project::Common::Error;
 using namespace phosphor::logging;
-namespace fs = std::experimental::filesystem;
+namespace fs = std::filesystem;
 
 void Download::downloadViaTFTP(std::string fileName, std::string serverAddress)
 {