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/image_verify.hpp b/image_verify.hpp
index d52281c..7ec0661 100644
--- a/image_verify.hpp
+++ b/image_verify.hpp
@@ -7,7 +7,7 @@
 #include <sys/mman.h>
 #include <unistd.h>
 
-#include <experimental/filesystem>
+#include <filesystem>
 #include <set>
 #include <string>
 
@@ -18,7 +18,7 @@
 namespace image
 {
 
-namespace fs = std::experimental::filesystem;
+namespace fs = std::filesystem;
 using Key_t = std::string;
 using Hash_t = std::string;
 using PublicKeyPath = fs::path;