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/static/flash.cpp b/static/flash.cpp
index 82c2393..9bb2bba 100644
--- a/static/flash.cpp
+++ b/static/flash.cpp
@@ -5,7 +5,7 @@
 #include "activation.hpp"
 #include "images.hpp"
 
-#include <experimental/filesystem>
+#include <filesystem>
 
 namespace
 {
@@ -19,7 +19,7 @@
 namespace updater
 {
 
-namespace fs = std::experimental::filesystem;
+namespace fs = std::filesystem;
 
 void Activation::flashWrite()
 {