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.cpp b/sync_watch.cpp
index c0f0f21..e4942fd 100644
--- a/sync_watch.cpp
+++ b/sync_watch.cpp
@@ -5,7 +5,7 @@
 #include <sys/inotify.h>
 #include <unistd.h>
 
-#include <experimental/filesystem>
+#include <filesystem>
 #include <fstream>
 #include <phosphor-logging/log.hpp>
 
@@ -17,7 +17,6 @@
 {
 
 using namespace phosphor::logging;
-namespace fs = std::experimental::filesystem;
 
 void SyncWatch::addInotifyWatch(const fs::path& path)
 {