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/sync_watch.hpp b/sync_watch.hpp
index 126c50c..d85ac8c 100644
--- a/sync_watch.hpp
+++ b/sync_watch.hpp
@@ -2,7 +2,7 @@
 
 #include <systemd/sd-event.h>
 
-#include <experimental/filesystem>
+#include <filesystem>
 #include <functional>
 #include <map>
 
@@ -13,7 +13,7 @@
 namespace manager
 {
 
-namespace fs = std::experimental::filesystem;
+namespace fs = std::filesystem;
 
 /** @class SyncWatch
  *